Because everyone has personality traits that don't become apparent until you've lived together for a while.
And because what you are suggesting is in fact, not that people make the hard decisions, but that, should an accident happens, they are not allowed to do anything to rectify it.
I've lived with my partner for a good many years, we aren't married, and I don't doubt that temptation has come along from time to time for both of us. But we chose not to follow that, not because of some altruism from a book, but because we love and respect each other. If we didn't, then the relationship would not work no matter what laws, pieces of paper of legislations were dropped on us. We don't plan to have children, quite frankly if I had kids, they would not get the upbringing I would like to give them because I can't afford it. I personally resent people like this Senator suggesting that somehow forcing me and Sharon to get married and have children will benefit us in any way whatsoever, though of course, in 18 years time, it does mean an unflux of free soldiers and taxpayers.