Weird. I wonder if you have to have a certain affinity for that because when my MC went down and saw Dawe by the pool, he was basically like "Meh, he's not bothering anybody" and left him alone lol. There wasn't even an option to do something else.
Even open relationships have lines that aren't supposed to be crossed though (as evidenced by the fact that Tommy is still cheating on Heather, even though they're in an open relationship, because he's not following the rules), and I'd imagine fucking a frat brother would be one of those lines. Especially knowing what we know about Tommy's personality.