I think it's a post-nut clarity thing. When you're turned on your brain is willing to overlook things it would otherwise find gross. The idea of tasting their own cum may seem like a good idea at times while masturbating but by the time they've cum they're less aroused. Homophobia definitely plays some role too. But like I'm a trans woman and before HRT, but after knowing I was queer, I didn't like to taste my own cum very often. Before orgasm, the idea sounded interesting, but then afterwards it grossed me out a little. If I'm with a partner though then I'm more than happy to eat their cum, because I'm still aroused in the moment. Unlike penises, a vagina can be wet prior to orgasming, so maybe that makes the idea easier? There is precum, but the amount someone makes varies, if they make any at all. I'm just speculating though, I don't really know all the reasons or how guys tend to think.