Homosexuals want the same freedom, subject to the same restrictions. To ask for a comparably, but not infinitely, broad choice of partners is not unreasonable. I think so, and today even the Vatican accepts that some people are constitutively attracted only to members of the same sex. By contrast, no serious person claims there are people constitutively attracted only to relatives, or only to groups rather than individuals. Anyone who can love two women can also love one of them.
Homosexuals currently have no marital option at all.
A demand for polygamous or incestuous marriage is thus frivolous in a way that the demand for gay marriage is not. Suppose, though, that someone insists that he can't be happy without several spouses, and that this is a basic constitutive need for him. Or suppose he says he can't be happy unless he marries a close relative. For argument's sake, let's say we believe him. The reason is that, from society's point of view, the main purpose of marriage is not, and never has been, to sanctify love.