For those who started off monogamous and then transitioned to ENM, what was it that made you?

My wife and I, monogamous for 15 years, have had discussions about threesomes and hard swapping with friends. We've never acted out on these fantasies and have reassured each other that we're satisfied as we are but we remain "open" to the possibilities.

My question here, other than fulfilling the fantasy, what real reason, if we're both truly satisfied, should we venture into the realm of ENM? Do couples who check all the boxes of their significant other still engage in ENM or is it that they feel there's something missing?

If I seem on the fence of the idea, know it's only because I take pride in my marriage and the relationship we share. We're best friends, partners and even gym buddies. Our sexlife, while not like it used to be, is still amazing for two people in the 30s. People who don't know us, assume we're newly weds because we still act like it.

I know these are questions only the two of us can answer but I'm curious to see where others from similar beginnings have found themselves...