Hi chocolatecookie (love the name!),
Here are a few pointers:
First, you might have miscategorized the story. Usually, the married-and-having-sex stories are placed in 'Loving Wives' not 'Group Sex'. That might confuse readers and get a bad reaction from ones who have a problem with sex outside the marriage. You might want to think about pulling it and placing it in 'Loving Wives'.
An information dump at the beginning is not the best way to draw in people. While there wasn't too much of backstory in yours, I think the story would benefit from cutting out the first paragraph and starting with the dialogue. This way readers get to know about the characters a little at a time and they would put together the information as they go along. The dialogue situates the characters in the action very well - we get to know the marriage is not working well and one friend is presenting options of how to spice it it up to another - all of which your little paragraph talked about.
Same thing with when the switch in pov happens. When you end one section with a cliffhanger and move to the next, it's a little boring reading through it. You tell us most of what we know already. Plus, in terms of the rhythm of the story, it's a horrible speed breaker. There's just been a sexual episode and now the reader has to wade through what was going on in Dom's head throughout the evening... and before.
Avoid block descriptions of characters. It might be a better idea to let the reader know how a character looks through the narration or through dialogue as the story goes its natural way.
In terms of characterization, I had the same reaction your first commenter did about James. He came out a little too uncaring. You might need to be a little nuanced with him.
Just a quibble - the cooking oil would be room temperature, not cold.
Overall, you might need to work the sex out a little more. Even the first episode was over too quickly, two paragraphs didn't even get into the mind of the reader. The threesome was good but will still help to stretch it out a bit.
The end seems a little hurried. There is too much happening in the last paragraph. It goes from the end of the sex to James dressing and leaving to Dom and Susan coming to terms with what happened. That's too much for a three line paragraph! A good way to think about this is that you don't need to state everything that happened. Maybe you want to gloss over some stuff and just state the important bits. The last sentence might work better as a standalone paragraph too.
I realize I mostly pointed out the bad stuff here but that was because you asked for what you could improve on and I focused on that. Your language is generally good and I couldn't find any typos, so that's good.
Hope that helps. Keep reading and writing!