The best reason to be married for a man (I am assuming happily) is that you will live longer from doing so. Sexual competition is a losing strategy for men from a living perspective, we know this from the life expectancy studies, including those of eunuchs who on average have always outlived 'complete' men.
The first comment is way way off; cheating was just as common in the past as present. My family history for instance has a man who abandoned his wife, moved to another country, married someone else without divorcing, then brought his sons over. It isn't a rare thing at all.
Second comment, men have an easier time finding casual sex due to women having more sexual partners. Big assumption, casual sex requires a casual partner. No woman interested in casual sex will choose a partner who is not the same.
In the past items needed for casual sex such as condoms were illegal and difficult to find, there was no pill so the consequences for casual sex were pretty big. It most definitely didn't stop it.
The rest I enjoyed, was funny seeing the train of thought that followed. The productive man thing was way off.
You can't talk about the changing role of male productivity without a mention of post war feminism, modern men only generate and control half of the productivity required to raise children. In the past couples use to work together too, but the roles were more restricted. Modern men cannot control a partner in the same way, this is where this argument was based.
Casual sex will always be more visible in an open society. Those more serious about sex tend to be partnered/dating or alone for long periods of time.
The why bother attitude from men tends to come from the serious men, not those chasing casual sex, those with easy access to it. They wait around until women begin to become serious about a partner. This is occurring later and later so some inevitably give up on women. The shortage of men thing has always occurred, in modern times for women it has never been better.
Historically for men it has always been fine to live as a bachelor. It is only in recent years as more misandry has developed in our culture that straight men are expected to be 'found' by a partner when she wants one.
For those of us that couldn't quite get into promiscuity, thank god for cam girls, porn, and the occasional date. Now we are almost hurt free.