A) Define "gay rights".

Using that term already implies discrimination.

B) As for discrimination:

Universal Declaration of Human Rights:

Article 1.
All human beings are born free and equal in dignity and rights

(...)

Article 7.
All are equal before the law and are entitled without any discrimination to equal protection of the law.
Now, I'll assume you're talking about gays being allowed to marry and I assume we all agree that gays are human beings.

Since non-discrimination is the rule, then not allowing gays to marry is an exception of that rule.

The question should thus not be "Why would gays be allowed to marry", but should be: "Why would gays not be allowed to marry?"

Answer that question first and then we can continue to discuss.