Argued many times. Societies don't grow unless there's some sort of incentive/punishment to steer men to marry and support families. Cheap prostitution, who in their right mind would get married? So, they keep any sex outside marriage illegal and immoral. Doesn't work perfectly, as we can see, but it keeps lots of stupid men stuck raising families. Most don't learn until it's too late, that we can get sex without the hassle of a house with a fat ugly wife and annoying kids.
I'm not saying that there aren't content married couples out there. But tell me the last time a guy who's married over a year, was in a hurry to get home after work so he could have hot nasty sex with his wife. Doesn't happen.
Single with a new girlfriend? Happens all the time.
You choose your type of sex. Same old same old, or new and hot (and more frequent).
That's a load of horseshit. Prostitution being legal won't change "love" or people wanting to live together. I'd argue that our country's hyper-puritanical view of human sexuality is one of the big causes of our high divorce rates. Society teaches us that even thinking of sex outside of a marriage is horrific and should never even be talked about. Couples can't even talk about that stuff without having to tread on eggshells and cross several bullshit and arbitrary social boundaries. Beyond that, life and partnership does not revolve around sex. Humans are sexual creatures, but we are also highly social and tribal ones as well. Maybe if we could be honest about this shit, and not bound by out-dated and worthless religious beliefs about sex, couples could be a hell of a lot happier.
I've known more than a few couples, straight and gay, with open relationships. It works incredibly well for them. Both partners are able to be honest about their desires and are allowed to seek adventures outside of the relationship, with some guidelines set by both of them. Obviously that kind of thing won't work for everyone, but it goes to show that sex being available outside of a partnership does not mean that partnerships do not happen.