Despite what you have seen, I have seen a little bit of both things. Some are like springs wound tightly and let loose: they explore the varied ways that humans express their sexuality with abandon and glee. Others: they go on as before, quietly enjoying their married lives and being monogamous.
Human sexuality is quite complex and defies pigeon-holing. Some humans, because of their makeup, cannot be monogamous, and some are quite fulfilled by it. Others are serial monogamists, others view casual dating and sex as their way of life. Some experience same sex unions, and return to heterosexuality, others prefer homosexuality. It's really too complicated to put labels on, and I don't think we should try to.
I prefer monogamy, and my husband does, too! Oprah had a show on a while back that had men speaking candidly about their infidelities. One man said that he believed that 99% of men cheated on their wives, and the other 1% were lying about it. I talked to my husband about this show, and he made a good point: the men saying and agreeing with this were cheaters! Hmmm...
Anyway, I believe it's perfectly healthy to sometimes fantasize about other people. Just because you're married doesn't mean you're dead! To act on it is another thing, unless both parties consent to it.
I would have liked to be monogamous with Gumby, but he's already taken... hehehhe.
CG