Well, for one thing, marriage has been redefined throughout history. It used to be a simple property contract where a man or his family traded property to a woman's family, for permission to marry her. It's evolved to an institution where people of different heritages can marry, childless couples can marry (without social pressure to have kids), and the marital status can be flipped on or off, like a switch, through no-fault divorce. All of this was accomplished without gay rights advocates.
Do you really think that the lack of an FMA constitutes "permission" for people to be homosexual? I remember the days when nobody talked about homosexuality (except to describe it as sin in a religious text), and gays were given near zero tolerance. If that wasn't "lack of permission" what would be? Yet, there were closeted gay people. They've always been in every society, they just get more mention in some than in others.
If you really want to fix heterosexual marriage, and in-wedlock birth rates, why is targeting homosexuals part of the solution?
who is targeting gays? it is they who are pushing this agenda to target what we now know of as marriage?
Why add a dysfunctional sex fetish as a solution? Wouldn't that be teaching the children to go the wrong way? Shouldn't we be reinforcing normal heterosexual behavior to strengthen marriage rather than belittling it and turning it into nothing but a bath house orgy?
I am reminded of an old Herblock cartoon of some politician or other lifting a paper cutout of a huge weight labeled "Balanced Budget Amendment", with an actual huge weight labeled "Actual Balanced Budget" off to the side, ignored.
The same applies here, with the paper cutout labeled "Homosexuals" and the real weight labeled "Divorce".