So if we were to 'invent' marriage, would it look the same as what we have today? Probably not, and that's an interesting topic just by itself. I know the Catholic Church "requires" it's members to go through several classes before getting married, and one of those strongly suggested that my wife and I not get married, because of my atheism, but those suggestions can be ignored (as we obviously did).

I think what you're suggesting only makes sense if you also prevent people from having kids without first getting married, in which case, you're giving this institution the power to decide who can and can't have kids, which probably isn't a good.

What marriage has is a mythos (probably not the right word, but I can't think of a better one) built up around it where it's all about love, and it's supposed to be forever, and stuff like that. You have a big, expensive ceremony, buy big, expensive rings, invite a lot of people, and generally make a really big deal about it. This, supposedly, prevents people from going through with it without giving it a lot of thought. It also means that you've essentially declared your intentions before everyone that you care about, and the fact that they now all expect the two of you to stay together helps motivate people to stay together.

Of course, not everybody goes through all of this, I think because people today don't really have respect for old traditions, but those traditions are there for a reason, and if we followed those traditions more seriously, then perhaps we wouldn't have the high rate of divorce that we have today.

Would I outlaw divorce? Of course not, there are certainly situations in which a divorce is necessary. Would I make it more difficult to have a divorce? I'm not sure. Everything comes with a cost, and you have to decide what's more important. If you make divorces more difficult, then you may end up with fewer divorces, but you also have more unhappy parents, which is bad in and of itself, but also probably not any better for the kids anyway. You could try to outlaw easy marriages, such as elopements, but you can't force people to have expensive weddings or pay a lot of money to get married. You could require counseling before getting married, but then whose going to pay for that?

I think what we have now is the result of balancing the various trade-offs, and it's not too bad. If someone comes up with an improvement, that will be great, but I don't have any suggestions right at the moment.

what about gays?

Your original question was, "If marriage did not exist, would you invent it?". I wouldn't invent it for gays, but I would invent it in an attempt to provide a stable home environment for raising kids.

As for the rest of your comment, maybe a marriage isn't necessary for deciding what to do with the kids, which is probably good since a lot of kids are born out of wedlock. However, I still think that if two people are planning on having kids, they should formalize their commitment to raising the kids, and having it be something cultural, like marriage, strengthens the commitment.