Theoretically all sims start as bisexual and then you can have them identify however you and they want based on their actions. You don't need to delete that sim if you'd like to have him come out further. He can be bi or gay, if he's woohooing with both genders then he's likely bisexual unless he comes out as gay afterward.
You aren't going to ruin his ability to be attractive to men if he initiates flirting with them at least at first. If you aren't planning to have him come out afterward then I'm not sure why you have him woohooing with a woman but it's your game and you can do what you want.
I've had gay, straight and bi sims in my game. I've been playing a lot of heteronormative households lately but that's because when I have gay sims in my game, I just "know". It's not based on stereotyping but it's just clear that they have a preference and unless they are bi, I don't have them flirt with the other gender. Jade Rosa and Ulrike Faust are both gay women in my game. They aren't bi and they just aren't interested in men like that. Other than the Heckings and IL couples, I've had 5 sims who weren't straight in my rotation so far. It's a fact about them like the rest of a sim's back story. They might have favorite foods, music, interest and they have orientations.
Several roommates in my game recently have been gay as well. I've treated them like all other roommates and rolled with it until they moved out on their own.
You don't have to play like me though.
