I will assume you really do not know RATHER than assume you simply parrot a leftist socialist village meme...
Simply put, because society has decided so. As evidenced in tradition, conventional wisdom, common law, and enacted law. Society -the people through elected representatives, in legislative bodies have enacted legislation that is premised alone upon the rational basis of procreation have decided such... Unlike those arguing for a leftist utopian socialist village ideology would suggest, marriage has never been accommodated, merited privilege, and rewarded simply to foster and promote love or even monogamous sex...
The incidental exceptions cited, e.g. your posting referenced above, do not negate the basis, they test it and in doing so clearly contrast against and specifically identify the basis that some such as you attempt to deny as one very much existing and relevant. Case in point, Griswold v. Connecticut where premised upon a right to privacy it was decided that individuals have the right NOT to procreate via use of contraceptives...
One can clearly see that with Griswold it is that contrasted with the exception that demonstrates clearly the rule, the rational basis of procreation exists!
Regardless a legislature has not chosen to handle exceptions to the basis, e.g. those that choose to contracept, exceptionally by incorporating more rules or by changing totally the basis or doing away with ANY marital accommodation -that is their prerogative, not something for the courts to decide.
Again, as evidenced in Griswold v. Connecticut, there is no supposed heterosexual (or homosexual) right to be accommodated and rewarded by society for entering into non-procreative marriage -such marriages are exceptions incidental to the basis of procreation with such incidence being premised in the right to privacy (just as abortion is).
It is only by illegitimately ignoring the rational basis of procreation - illegitimately conflating the right to privacy (which prohibits the State from enforcing procreation) with the privilege accorded marriage (rationally based in procreation as provided for legislatively by the State) that one can even attempt to argue the ability to choose to engage in homosexual sex with another as something that merits anything from society.
In essence, homosexuals do not get a "free pass" under the privacy right like non-procreative heterosexuals do BECAUSE homosexuals objectively can not possibly ever procreate homosexually...
The ability to procreate and the possibility of procreation -something two homosexuals can not do no matter how much they try...
I do not "parrot leftist socialist village memes." I merely reject the socialist notion that there's this "society," and that free, sovereign individuals should submit to this "society's" every whim.
You said nothing of substance in your lengthy response, but merely hit the usual conservative buzzwords (society, procreation, privacy, Griswold, yadda, yadda), which amount to nothing more than saying it takes a male and female to procreate. Yep. And then what? Males and females have procreated for thousands, even millions of years, with neither "societal handouts" at the expense of the non-procreating, nor legislation that is "premised alone upon the rational basis of procreation."
You explained THAT governments have been poking their noses into marriage for centuries (as if I didn't know), but you haven't explained WHY they should continue to do so.
Very good explanation. I think many of us realize this on an intuitive level, but you express this in words nicely. Thanks.