Why do we measure how "good" a Christian is based on how far away it is from "pure" Christianity? Doesn't all of Christianity become silly at this point?

New York's legalization of gay marriage got me thinking even more on a question I've been trying to work out for a couple weeks now.

Christian supporters of gay marriage, or at least "live and let live" Christians are given a tentative show of support by the Atheist community, to show what tolerance and respect can do for society. But these "Good" Christians, so to speak, seem to be measured by how far away they stray from the fundamental Christian doctrine. For example, a gay christian will be vehemently ridiculed by fundamentalists, but a few moderate Christians would still support his cause and still accept him.

Let's be clear, the bible explicitly forbids homosexuality (I in no way condone this), so I guess my bewilderment comes from good and kindhearted people who simply remain complacent from the old, antiquated system that is Christianity