If gay people really want to get married, all they have to do is to become straight and marry someone of the opposite sex. There are several problems with this argument, the first of which is that it presumes that sexual orientation is a choice. This lie is promoted so endlessly by bigoted religious leaders that it has become accepted as fact by society as a whole, and it was advanced, beginning in the 1980's, for the purpose of discrediting the gay rights movement. But the reality is that a half century of social research on this subject, consisting of thousands of studies, beginning with the Kinsey and Minnesota Twin studies of the 1950's and continuing to the present, has shown conclusively - beyond any reasonable doubt - that among males, sexual orientation is only very slightly flexible, and among females, it is only modestly more so. That homosexuality is congenital, inborn, and has a strong genetic component. In other words, if you're gay, you're gay and there is little that you do about it, regardless of the endless propaganda to the contrary.
Another problem with this argument is that it presumes that heterosexuality, if it were a choice, is self-evidently a more desireable and/or morally superior choice to make. This is a qualitative argument with whom many gay people - and many thinking straight people as well, both religious and secular - would take issue.
A third problem is that this argument presumes that someone else has the right to veto your presumed choice sexual orientation on the basis that they are not comfortable with the choice you have made. It is difficult for me to see how any religionist or anti-gay bigot, however sincere and well-meaning, has the right to arrogate to himself that veto power. Or, frankly, why a homosexual should be forced to go out of his way to make bigots comfortable with their bigotry.
A fourth, legalistic problem with this argument is that it presumes that if the choice of sexual orientation can be made, the voluntary nature of that choice removes any and all right to the protection of the law for the choice which has been made. But I would point out that the First Amendment to the United States constitution protects, by constitutional fiat itself, a purely voluntary choice - that of religion. So if it is acceptable to argue that unpopular sexual minorities have no right to equal protection of the law because they can avoid disadvantage or persecution by voluntarily changing the choice they have presumably made, then it is equally true that the First Amendment should not include protection for choice in religion, because no rational person could argue that religious belief is itself not a choice. In other words, this is like arguing that you should not expect legal protection from being persecuted because you are a Mormon or a Catholic; the solution to such disadvantage or persecution is simple: just become a Southern Baptist or whatever. I have never, ever seen a religious opponent of homosexuality who is asserting that homosexuality is a choice, advance that last point with regards to religion - a fact which very glaringly demonstrates the clearly bigoted character of this argument.
http://www.bidstrup.com/marriage.htm