I think your question should be "Why do (some) men and (some) women....etc
You really can't generalize either sex.
Anyone who has ever seen an episode of "Cheaters" or "The Maury Povich Show" can tell you there are women out there who are having sex with more than one man.
There are countless reasons why someone "needs" to be with more than one person. It can be something as simple as being too immature and not ready to settle down, the thrill of having variety, something is lacking in their primary relationship, feeling unloved, unappreciated, or an unbelievable opportunity to be with someone out of their league presented itself...etc
It's really a waste of time trying to figure out why the opposite sex or even a specific person does what she/he does. The only person you will ever have control over is you!
All you can do is try and find someone who wants what you want and then treat each other like gold!