The same question about some men, though. lol.. As a woman i am failed to understand why some people from both genders do that. I know some people who sleep with more than 1 (of course without notifying their partners), and they came with different and varied reasons.
One friend, she slept with 1 (that i know) other guy, because her husband who is much older than her is always overseas for work, they didn't spend adequate time together. She's quite attractive and young. Later she found out that her husband's cheating on her, remarried with another girl in her country of origin. I am not sure who started first, but it ended up with she cheated on him as well. In this case, a young attractive woman, busy husband with automatically much less sex, lonely, some man were flirting on her, i think that's enough for SOME people to cheat on their partner.
Another friend, she has a husband with a higher level of sexual-drives..while she can't really play tango with him since they share a different pace and energy for sex. She had a feeling that he would play with other girl but---I hardly understand---she seems to be loose about it, like no heart-feeling. On her side, she once told me that she made a 'distance-friendship' with another guy overseas..and she seems pretty excited with the new 'friendship'..but she hide it from her husband and said "well, why not..we're not going far like meeting in person, it's only on internet. And i never know either if my husband does the same with others outside without notifying me.."
So, from both story, you can't really give one answer. Yes, the second story is not about 'sleeping', but to me the red line of all is the relationship between 'trust' and 'behavior'. But other than that i would say there are some people who are 'sick'..and really do what you are questioning..
Hope you'll get somebody throw some psychological insight about it..it is interesting to study the 'in-depth-why'..