I'm surprised by the number of nay-sayers here.

Obviously convincing someone of Atheism is better than convincing them of the acceptability of homosexuality, but that doesn't mean that the latter is bad. The comparison to swaying an Astrology believer (saying they're an Aries, not a Taurus) is not apt; OP's idea is not to trick them using theist arguments, but to point out the intellectual bankruptcy of believing both in tenets A and B. As a few of the others here have pointed out, the way to remove the wooden beam might be by removing it sliver by sliver.

I think a site such as this would offer an excellent resource for Atheists/Agnostics who regularly argue with Christians over this topic; during my own deconversion process, I was greatly aided by the likes of Professor Richard Carrier who did the scholarly work and laid out the arguments for me - it saved me months of time spent researching, reading, and seeking out sources.

One argument, OP, that I hope you address - I heard this from a bartender the other day. "I'm against gay marriage/adoption because a child needs influences from both sexes." A trite, easy to understand response to this would be wonderful (I'm sure everyone here knows the answer, but I'm trusting that knb will be able to write it with eloquent rhetoric).