Grace for Gays?

To me, the rejection of the Christianity of gay people is similar to the rejection of the Christianity of Mormons.  Traditional Christians reject Mormons Christians for their rejection of orthodox formulations of Christian doctrine/dogma, they reject gay Christians for rejection of traditional behavioral norms.

Being new to traditional Christianity, I have some serious questions about how the Christian community currently rejects/embrace Christians who live non-traditional lifestyles such as gay marriage. If you have some time, let me know your best thoughts on these:

(1) What is the most compelling Christian theological justification for classifying sin such as homosexuality as more or less abominable in the eyes of God?

(2) Is requiring heterosexual practice in order to accept a person into a Christian fold any different in principle than requiring circumcision?