It's clearly not according to me.
I wasn't alive during the last few million years where everyone else decided what marriage was going to be about. I wasn't in all these different cultures all over the globe.
This is basic Sociology. The key term is "cultural universal". No matter where you go, marriage is about socializing children just like funerals are about marking the end of life.
The problem with your argument is that it does not allow for society to examine how it is doing something and why. It relies on the concept of "this is how it has always been done, everywhere". Not only is this wrong, but it also doesn't actually examine why we actually get married, when we decide to get married, or who we decide to marry.
The reason saying that "it has always been done this way, everywhere" is wrong, is because although almost, if not all, societies have only had heterosexual marriage up til the past decade or so, most societies have not limited heterosexual marriage to just be for raising children. Some cultures and/or religions might still make such limitations, but they would be very few.
Also, although there is some biologically instinctive attraction involved with picking a spouse, it is not the rule that a person will fall in love and wish to marry the person that they feel that they would make the best children with as a couple. It is also not the rule that a couple always chooses the most beneficial time for the child, to start having kids. Most procreation happens, whether planned or accidental, because the parents were thinking of themselves, not the actual welfare of their offspring (and I don't want to be taken wrong here, because I don't think that this makes most people bad parents, the most ideal time may never come if you just wait for it, so sometimes you just have to make the most of what you have).
Many people do not get married to someone because they are ready to have children with that person. Many marriages happen with little thought about when during that marriage children will be brought in, if ever. And many people who can't produce children with each other, have happy and fulfilling marriages for themselves.
Legally recognized marriage in the US is what we are discussing, and unless the US government declares why they only recognize heterosexual marriages with a sound reasoning behind their explanation and marriage laws that reflect that reasoning specifically, then the US government is discriminating against homosexual couples by not federally recognizing homosexual marriage and enforcing the Full Faith and Credit Clause.