Do you like the situation that he has a girlfriend, as in, you'd like to have sex with the guy or something in front of the girlfriend or with her somehow involved, or is the "masculine factor", like brandonizer said, they are more masculine? I'm just curious
I'm a woman who likes to see guys together, so I prefer dating bi guys and like seeing them have sex with other guys. Some gay guys tend to shy away from the situation, not because of the woman watching, but because it feels "straight", the same way as many straight men don't want to be in a situation where there's another man involved in anyway, because it feels "gay". (And of course for some it just doesn't do a thing to have a woman watching or anyone for that matter, but I just started to think about this because you mentioned guys with girlfriends
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Anyway, just wanted to say that even liking some girls doesn't change gay to bi, just makes them homoflexible maybe. At least in my opinion to be bi means a person looks at boys and girls both the same way, for sex and love, even if they'd have a preference