What ever happened to "sexual preference?"  The homosexual activists canned the term because it expressed the truth; "born with immutable sexual attraction" is an easier fight, especially when it's impossible to prove a negative.

I'd probably stick with a logical argument about anatomy and physiology (as none of my gay friends are religious) and how they are missing out on the greatest blessing that we have as humans: a complementary sex with whom we can create offspring.  Such an easy trade-off for perverted use of the sex organs.