The more I think about it, I don’t think Phil is gay, more than he’s a sexually repressed, aromantic, asexual man-child who thinks any form of affection is icky or something to be ashamed of. Where this comes from I have no idea. His mom? His upbringing? Lack of experience? He’s a Freudian mystery to me.
Pure speculation, but I think lack of experience is a big part of it. We've seen how Phil gets with multiplayer games, both when he's going against an opponent, and also in the rare times he's playing with other people. Phil wants to be the top of his team, going "beast mode" and carrying everyone because he's the best. Even outside of multiplayer gaming, he makes excuses if he's struggling, and insists that "if he really tried, he could be the best at a game, but he doesn't care." or similar things.
I think that same logic applies in the bedroom, except he couldn't blame Leanna or Khet for his failures. Dude doesn't take care of himself, has minimal relationship experience, is incredibly selfish and given how much he drinks, may very well have a case of permanent whiskey dick. Leanna especially also showed she wasn't afraid of teasing or outright mocking Phil. Again, this is just speculation, but I always figured he was just a massive disappointment with her, and she didn't hide it because, especially as their relationship went on, it felt like she got more comfortable telling Phil what she wanted/needed.
So yeah, Phil sucks at sex. His ego can't handle it, and he can't blame the other person to deflect responsibility like he usually does. I dunno if I'd go so far as to say he's asexual, I mean, we have proof that he'll play with his joystick, I think he's just terrified and insecure about sex. He tries to play up the "maturity" angle to take the high ground and justify it, just like he tries to play the "raised catholic, charity is private" card to take the high ground when it comes to his generosity.
He has a fascination with big buff dudes, and there's definitely a chance it's an actual attraction, but I think it's idealization. We've seen Phil associates the people he watches/plays as being him. He wants to be this massive body builder tough Italian mobster guy, so he tends to show those characters deference because in his warped mind, insulting them would be like insulting himself. Buuuut, it definitely isn't a stretch to say idealization can lead to actual attraction. Even if he is gay, I think he'd still be just as timid and afraid of sex with a dude as much as he is with a girl. Probably even more so, because he definitely thinks being gay is something to be ashamed of/a punch line to a joke. Dude was raised on 90's sitcoms and movies and has been making the same gay jokes for years now, and I don't think there's ever been like...any lustful undertones in the jokes. He just thinks butts/poop are the height of humor.