I don't like incest so I doubt I could evaluate this objectively and I didn't read the whole thing.
Here's an excerpt:
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It all started on one of those particular days in fact. My girlfriend at the time had just stormed out of the house after I had, what she thought, gone too far. We were making out on the couch watching, or not watching, TV when I started rubbing her crotch. At first she did nothing and I thought she liked it, but all of a sudden she pushed me away and slapped me, calling me a pig and then leaving the room and my house. At first I was mad and wanted to chase after her, but I knew it wasn't worth it and sat back on the couch, thinking about my increasing horny state.
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You've got a tense change in the middle of a sentence("...slapped me...leaving the room.") The "...I had, what she thought, gone too far." is awkward. ("after she thought I had gone to far" is better)