First off, this is pretty damn good. I think you have enough understanding of both the art and craft of writing that you could try for publishing. So keep writing. =)
My first problem I had with the story was just how... luxuriant the descriptions were. Setting the scene is all well and good, but the story feels like it's at least a third longer than it needs to be because you just couldn't rein yourself in description-wise. Too many adjectives, too much back story, too much flashback. And all that's happened so far is that Raven has gotten into her car. I'm a third down the first page and I already feel like things are moving too slowly.
Secondly, if Brian's sneaking peeks at Raven's ass, his situation with Constance is already on the rocks. I feel like you could stand to draw more attention to that--to either have Brian present more of his problems with her earlier, or be deeper in denial about it.
The story is in present tense, but you switch into past tense for almost all recollections--"My parents ate dinner." To me, there are certain ones that would be better served by past perfect aka pluperfect or present perfect instead ("My parents
have eaten dinner"). For instance, when you're talking about Raven's parents' divorce, that's a good time to use past tense, because it's back story, ancient history. But when you talk about Brian woolgathering for an hour and claim that "Constance left to go home at that time," it's
also in past tense--back story, ancient history. Using pluperfect--"Constance
had gone home"--would help you differentiate between the recent and non-recent events.
(Before you start crying and complaining about grammar, let me point out that this is a
really good problem to be having. Which would you rather be hearing--these finicky nitpicks about minor nuances of meaning, or that "bar" has only one R in it?

)
Okay, here's another example of the whole "too much description" thing: the "Lucky Charms" incident. First off, it drags The Reader out of the narrative by suddenly throwing back story into the mix. Second off (and I haven't even started reading that paragraph yet, so bear with me), does it actually
need to be described? "Incident with a charm bracelet" makes it clear where the nicknames came from, and sometimes things are best left to The Reader's imagination--hence the concept of the
Noodle Incident. (Note: do not click
any links on that page if you intend to do anything productive for the next several days. TVTropes will eat your life.) Finally, if it's really important, you can always revisit it later: leaving plot threads hanging is a good way to keep The Reader curious and invested.
(And no offense, but the whole snippet seems really irrelevant now that I've read it.)
Constance saves Brian's life from an allergy and that's what makes him want to marry her? Sorry, man, but I don't buy it. Reverend Hastings surely did pre-marriage counseling with them, and he would have emphasized that it takes more than just coincidental stories to found a working marriage--right? I mean, even the most seriously conservative priests do that--and I almost married a real-life version of Constance, so I'm not just saying that. It's cute, but Brian had better be able to offer better compatibility than "She saved me from an allergy" if you want The Reader to believe he and Constance actually get along.
Also, the stuff about Rev. Hastings' hypocrisy feels like a cheap shot. I know from your postings here that Constance and Brian are not going to work out. Well, these things happen sometimes. They're not right for each other--that doesn't mean she or her family are required to be bad people. And the way it then elaborates out... Obviously, Brian is young, but not
that young--he ought to know that marriage doesn't change a person, that getting Constance to say "I do" won't solve their pre-existing problems. (And if he didn't, Rev. Hastings ought to have told him so!) If Brian married Constance
knowing that her father is a racist control-freak, then he's either crazy or wildly optimistic. And with that in mind, I feel like he gave up on Constance very abruptly. There ought to be history--arguments they've had which are now only being rehashed--but none of that is mentioned. Evidently, all this controlling and racism and such is new to Brian. That just makes him look stupid.
And yes, the ending is extremely abrupt. More importantly, it seems to be a bad one, since the final thought of the story is negative: "Aww crap, there goes everything!" And because the story doesn't continue, so far as The Reader knows, that
is everything: Constance is over,
Raven is over, and Brian's got a rocky road ahead of him. A "To be continued" would have been incredibly useful.
Now, as to your specific questions:
The reason readers can't figure out why Brian never pursued Raven is because you never had him talk about it at all. The sole excuse was that Brian's in The Friend Zone, but more importantly he seems to be
okay with being there. He has one incident of noticing her ass, which in itself means nothing--women are women, and guys sometimes notice that one of them is sexy even when that woman is supposed to be off-limits (best friend; incestuous content; etc). This is normal, and--because the vast majority of men are sane--these moments pass on and are not remarked over. Long story short, the reason readers are confused about Brian's attraction to Raven is that he doesn't seem to
be attracted to her. There's nothing about their personalities or chemistry, and no indication that he is having any attraction to her beyond a fleeting notice that, hey, she has sex appeal.
As to what Brian sees in Constance: to be honest, I haven't the faintest clue. Brian doesn't talk about
that either. Does he like that she's strait-laced and traditional? Does he like her spirit of rebellion and her interest in starting to break free of her parents? That second was the core of my attraction to my own version of Constance, so I can vouch for its power. Do they have any other interests in common? My Constance--let's call her "Caitlyn"--is a musician and a writer, same as me, and I also had to fight a long campaign to break free of my parents, so I empathized with her plight and counseled her on it. Of course, she also wanted me to be much more devoutly Christian than I am, and I wanted her to be much more devoutly
sexual than she is, and we couldn't compromise about it (I don't have much faith in organized religion, and she, poor thing, is so asexual that sometimes just making out would give her panic attacks). And boom, that was our chemistry in a nutshell--why we fell in love, and why it didn't work. What was Brian and Constance's? I don't know. I hope you do, because you need to put it in the story.
And in regards to the whole escaping-her-father thing, the reason readers don't get it is because you have the timing on it reversed. You do a lot of back-story on Brian's past, particularly with Raven, in the early stages of the story, but don't bring up his relationship with Constance's father,
her relationship with her father, Brian's opinion on what it should
be, until, basically, the wedding itself. All that stuff ought to be right there in the beginning, alongside or maybe even replacing the other descriptions. I know that you're following the Law of Narrative Importance--since Raven is ultimately the girl Brian ends up with, you talk about her first. (
First Girl Wins.) Unfortunately, in this case, Narrative Importance leads to bad characterization: Brian, who
should be dwelling on his concerns for his upcoming marriage and the war he's going to have to fight against Constance's father, is instead dwelling on his best friend's ass. Obviously, that's on purpose, but you could be doing a
lot more to make Brian and Constance look like they actually have some sort of chance together. Because don't fool yourself: Constance is
choosing not to sleep with Brian, is
choosing not to move her relationship with him forward, is choosing (let's be honest here) her father over him. That's a valid choice for her to make, but the fact that Brian doesn't at all try to fight it seems (as previously mentioned) abrupt--particularly with the lack of foreshadowing. He should have more reaction than "Whatevs, I'm out". This should be something he's been agonizing over for ages. "How do I initiate her into sex?--because I love her and I want her to enjoy it. And what the heck am I gonna do if she doesn't decide to give herself to me?" And that's more than a figure of speech, since Constance is balanced on the cusp between her old life, the life her father chose for her, and the new one Brian is trying to draw her into.
Your instincts are right in feeling that the central question of the story is, "what the hell got Brian to marry Constance and then give everything up for Raven". The problem isn't so much that you haven't answered it yet. The problem is that you haven't
asked it yet. That's why readers are having the responses they are. You simply establish that Brian married Constance and then gave it all up for Raven, without bothering to have
the story ask why or even, really, suggest that it is
going to. Brian does far too little self-reflection for that to happen. So ironically here I am complaining about all the back story and then simultaneously claiming that you need to add more description. But seriously, that's what it's about. I want to see a lot more of Brian going, "What am I doing? What am I doing? And, more importantly,
why am I doing it? Why do I like Constance? Why do I like
Raven? What makes one right for me and the other not?" It's okay to save most of the answers to those questions for later, but you need to at least start hinting--and you
definitely need to have Brian start
asking.
Okay, that's probably enough to be going on with. (Took me 90 minutes to write this up.) Be careful with the TVTropes links--that site kills productivity like no other, especially for writers. Hope all this analysis helps some. And again, please don't be daunted--
These are good problems to have. "I'm not entirely sure Brian's motivations are consistent with his actions." That requires your story to 1) have characters with 2) discernible personality. With those alone, you're already a really good writer.
Now let's make you better.
