What is the real reason for opposing gay marriage?

Most of the arguments I’ve heard against same-sex marriage boil down to either diluting the value of marriage or that marriage is all about having/raising children.

On the first count, it seems to me that a real marriage is founded on two people loving each other enough to make a lifelong commitment, and that if you want to go with the traditional view of marriage then it’s solely about property rights and thus more of a commercial transaction than a religious institution. Not to mention that it’s hard to enforce such anti-homosexual discrimination without running afoul of the First Amendment protections on freedom or religion and (by extension) the freedom to have a different belief system that allows two consenting same-sex adults to be together.

On the second count, does that mean that heterosexual couples that have no desire or ability to reproduce should also be barred from marrying? What about couples that merely cannot afford another mouth to feed; are they also prohibited from tying the knot? And where is the proof that same-sex couples who raise kids “turn their kids gay”? Most of the gay people I know had straight parents, and the few people I know that had two same-sex parents are all straight.

So tell me, what is the REAL reason to perpetuate such discrimination?