Oh... Sorry to poke a dead stick with a horse then...
While I completely agree with you (especially IRL), there are two points I would like to make. The main one being that there are a lot of people who are not a firm 1 or 5 on the Kinsey scale, she could be a strong 4-4½. The other being that (as far as I know), no one is trying to make her straight, slightly bi at most. As a bonus point, she does seem to have a small amount of attraction to him (or at least enjoys flirting with him), even if she would never choose him under normal circumstances.
The thing that bothers me most often on this topic is that we seem to forget that there are bisexuals, we ignore that everything isn't all or nothing, but most things are shades of gray (even if it's a pretty dark gray in this case, if at all). On TV, you have a character who spends her whole life attracted to men, she finds she is attracted to a woman and suddenly she is gay. That might be how people look at it, but that is not how it works.
As for your question 'why', it's a simple male fantasy, being so skilled as to have someone willing to make an exception for them, gay or straight, it's a common male fantasy, comes with the testosterone I guess.
If this were not a game, I would be condemning the MC, not just for her, but for all of his night time activities (as well as many others, it's pretty much all shady), same with most porn games, but it's fiction and she is pretty much the best looking woman in the game. In this situation,
not doing something with her would be a crime. ...Sorry, I couldn't help it, always try to go out on a joke.