Him flirting with another guy while in a relationship with her might hurt his likability. That is, if they haven't had a good reason to hate her yet. We want the readers to be on his side. I would say during the relationship would be better than before. Maybe at some point she goes through his old stuff and find hints of it? A former boyfriend, certain pictures that a straight boy wouldn't necessarily have, an undelivered letter to a guy, an online blog where he talked about it, someone at school throwing it up in his face… It really depends on how he feels about his sexuality. If he tries to hide it, if he doesn't deem it important, if he's embarrassed by it, if he's in danger because of it, if people already know about it and he never really thought about discussing it with her.
Sorry, I'm getting a bit carried away with it. lol