I can't imagine going to my theorerical wife-in-open-monogamy-relationship and tell her that the girl I had sex with at work's Christmas party gave me std because, despite she had her pills, but the rubber fell. It's just not mixing up in my head.
Also, if I give myself to my wife as a whole (i.e. I take care of her, the home and the children) I do not have time really to have another affairs. The rest of the time I'm left with I either sacrifice to be with her or have my own time like play games or compose music. There are lots of things to be done really, and I couldn't imagine sacrificing my family and duties to pursue sexual satisfaction with other people outside of my family.
> I do not have time really to have another affairs.
That's a good point, and true of most people I expect. The wealthy have time, though. Wealth buys time. Kings, the epitome of being wealthy historically, have always had time for affairs, and are oft remembered for exactly that(!), but a growing proportion of "common folk" are becoming increasingly wealthy themselves, freeing up the necessary time among more and more people. However, wealth inequality is high, so the opportunity of time is not evenly distributed.
Also, if I give myself to my wife as a whole (i.e. I take care of her, the home and the children) I do not have time really to have another affairs. The rest of the time I'm left with I either sacrifice to be with her or have my own time like play games or compose music. There are lots of things to be done really, and I couldn't imagine sacrificing my family and duties to pursue sexual satisfaction with other people outside of my family.