There is no shortage of men who are unfaithful to their SOs. But we largely expect women to be better than us. And it is always somewhat shocking when they are not (but it probably shouldn't be).
My first real introduction to how two-faced women could be was during my senior year of college. For most of that year, I had this fuck-buddy who was engaged to recent grad. Her fiance's job was a few states away, but she was still a junior. Anyway, I met her at a party and before too long were going at it nearly every day. I had no desire to marry her, so she kept the whole thing secret from her fiance (who she insisted she loved more than anything else). That was bad enough. But a few times we were at her place and she would talk to him on the phone while we were doing it. She really got off on it, especially when she was blowing me between replies. So much for true love. After I graduated, we lost touch, but I do know she married the guy, pumped out a few kids and is still with him. So I guess it all worked out for her.
Since then, I've run into many jaw-dropping examples of female infidelity and betrayal. Chinese women seem to be the worst offenders. Many Chinese women with husbands (even American ones) seem to think nothing of cheating on them. It's almost a given that FOB Chinese women will fuck an American man, regardless of what relationship she has back home. And, likewise, they love the phone trick too.
We can debate why society expects women to be better behaved sexually, but I suspect it all comes down to biology and evolution. Women are the guardians of reproduction and it is absolutely critical for the husband to believe that their kids were sired by him. Though DNA studies have show that is not true about 30% of the time.