I have an older brother who has been happily married for 30 years. His happy marriage never precluded him from playing on the side at some point. The friend of mine that I mentioned in another thread that got busted when his mistress messaged his wife also loves his wife intensely. So inasmuch as there are some here not enamored with their spouse or married sex life, there are also men that truly love their wives but want the variety of sleeping with other women.
In a committed relationship, it's very hard to keep that charged excitement of sleeping with someone for the first time. Sex and love are two separate entities, even though we like to conflate the two. An SP is ideal for sex because it's not a messy relationship. It's the opposite of my buddy that got caught because he was also emotionally involved with his mistress.
As already discussed and said well by EB, I don't think human beings are naturally monogamous creatures. In sexually reproducing animals, monogamy is the stark exception. But I'm also aware that unlike animals, we're moral beings.
Big Nat, good luck with your decision. Two things that are important:
(a) if you do see an SP while being married, don't pass on a STI;
(b) do NOT in any way suggest opening your relationship. It's too fraught a move. She may perceive your request to open up or do it as "a team" to mean she's not adequate enough for you. And if she says no, she's now on alert that you might stray. And if she says yes, it's going to be a case of unintended consequences because a woman can get sex readily more than a man can. The only way suggesting an open relationship is worth the risk is if she's already made it known she's attracted to women or hinted around seeing other people.