There has been no study performed that indicates that heterosexual or otherwise "traditional" parents have better success with their children than anyone else.
educate them in better eating habits and why!
No, not normally. If they are arroused by their own bodies, they are narcissistic (but straight people can also be narcissists). Gays are no more likely than heterosexuals to be narcissistic. They want a partner to make love with just as heterosexuals do.
Heterophobia
No. Worldwide, MANY more heterosexuals have HIV and AIDS than gay people.
Proof to them that is better there than here.
Do their best. Do better than their parents. Be complimented by someone else. Be healthy. Be happy. Be helpful. Be good.
heterosexuals
No, both are heterosexuals.
get better grades
No. If your parents love is unhappy that should be your mirror that you will not make the mistake again. Make your marriage better than them. Besides that is your choice if you let your parents unhappy marriage affect you.
Voice is out of one's parents, you 'd better take good care of your throat, drink more water.
Just tell them. There is no way that stalling will make it better.