Maybe you shouldn't tell them. Or at least fin a more subtle way to make them understand you're not interested, more subtle than basically telling them they're too ugly to match your standarts...
No, you're not gay
Being gay means being attracted by men, you could enjoy being fucked by a women wearing a strap-on, it wouldn't mean you're gay. Do, women ask themselves wether having their pussy licked by a man means they're lesbians ?
Homosexuality is about feelings, about attraction, not about sexuality so as long as you're not attracted by men, you're not gay, whatever you may do and enjoy (or not) in the bedroom (or anywhere else).
Now, to help you with that sex issue, my advice would be to use your past experiences to guide her, tell her the kind of moves your like (including during the act itself). Remember to encourager her, particularly when she's doing better (even if you think she could do better).
And don't forget to reward her by going down on her too...