Homosexuals are one of the few minorities who actually make a conscious choice that that is what they wish to be.
I am afraid that homosexuals do not make a choice to be sexually or romantically attracted to the same gender. They can only choose to act on their desires or to resist them.
Much has been made of the recent study by Robert Spitzer on homosexuals who appear to have changed their sexual orientation to heterosexual. It is true that many of these persons have succeeded in living in heterosexual marriages, but only a minority of them reported no longer having same-sex desires. Moreover, the sample of individuals Spitzer studied were not randomly selected; they were volunteers. Spitzer has made it clear he believes that the large majority of gay men who try to change their orientation through therapy do not succeed.
Accordingly, I think there is a good case, based on compassion, to have limited legal recognition of same-sex unions. The only other reasonable alternative is of course celibacy (chastity) for gay men: but let us be realistic, most men are unable to follow this path. They may attempt a heterosexual marriage, which would have a high probability of failure (an arrangement not fair to any woman!). If they do choose to act on their homosexual desires, far better that they do so in the context of a caring, stable relationship. (Given that 25% of sexually active urban gay men have HIV, any reduction of homosexual promiscuity is to be welcomed. There is a minority of gay men who are in long-term monogamous relationships.)