No, it's not. It's about money.
It's about spousal benefits for insurance, retirement, social security, and other entitlement/benefit programs.
Government and society grant certain benefits to married couples because, as stated earlier, most cultures and religions have recognized marriage to be the best method of perpetuating the society. (Even though not all marriages result in children, odds are that many or most will.)
Homosexuality by definition does not perpetuate the society, which is one reason most religions and societies have prohibitions against it. Homosexuals want the benefits married couples get, but they don't benefit society in return.
Actually, I don't believe they would even be able to demand marriage or civil unions if not for the current cultural decline, and the fact that "the joining of one man and one woman and the binding force of the nuclear family" is much weaker than it used to be.
Many couples now choose not to have children. Where I teach, very few of the children live with both parents, and some don't even have fathers listed on their birth certificates. We can't really claim that marriage perpetuates the society, because so many of those who are perpetuating the society aren't remaining married.