Some people tend to monogamy and others to polyamory, and some to none of the above.
I'm actually monogamous myself, I have one sexual partner IRL, and everything else (such as my activity here) is just fantasy material for masturbating.
If given the opportunity to have a session with a sex worker IRL I would refuse as long as I am still in a relationship with my partner.
One of the factors in my monogamy is the golden rule: I would not want her to have other men, it would make me jealous - I'd fear losing her to them, so I wouldn't do that to her either even if I had the chance.
As for polyamory and such becoming more common, if that's even actually the case, it's probably because people are hearing about it now and those who would naturally tend that way now know it exists and can feel better about trying it.