Hi Sea, I think we need more information in order to give you more than the most generic advice. Flesh the situation out for us. Like, do you have any sort of friendship with the wife? Is your relationship with Mr. Married Man open to growing into something bigger or is it clear up front that your role in each other's lives is as secondary rather than primary partners? Stuff like that.
Me, 30ish bi female, been doing solo poly for roughly 5 years. Gia, Clay, and Pike, my partners. Davis, ex/friend/"it's complicated." Eric, Gia's husband. Bee, Gia and Eric's toddler.