There are cases where gay is a choice, but likely not a conscious choice. For the majority of people who identify themselves as 'gay' they were born this way. There have not been a lot of medical studies on this issue, mostly because people who are gay don't want to participate in anything that might lead to prenatal (or postnatal) culling of gays due to a gene or mutation identification. Very valid points!
I will say, though there is one study you should read and show your friend:
http://www.time.com/time/health/article/0,8599,1815538,00.html
It indicates that the brains of gay men more closely resemble that of a female than a straight male. Is that genetic or developmental? Either way, it's not a choice. That much is very clear.
Your friend sounds religious, so here is what the Catholic church says about these issues (although your friend may well be anti-Catholic, in which case, this won't help you) Every person is made by God and loved by God. Gays are made by God and therefore are loved by God. Having gay sex is wrong, not being gay. However, every person is called to celibacy except in marriage. So, in fact, any sex that is outside of marriage is wrong, even heterosexual sex, because we are called to be better than our base needs.
Essentially, from a religious perspective, your friend has no right to condemn gays, as they are God's children as much as he is. They should be welcomed into the congregation just like any other person. However, if the people in question are sexually active, a congregation should look upon it as a sin, in the same way they would for a hetero couple having premarital sex (possibly even living together). It is the same.
As for marriage, most churches believe God called for a man and a woman to leave their parents and form a new family with the intent of having children. This is not possible for gays unless they are unfaithful to each other, which is not giving marriage the sanctity it deserves. That said, if gays wish to be married and are not following a faith that denies marriage to gays (or are not religious at all), then ask your friend to explain how allowing a legal union between any two people is detrimental to society.
Good luck.