Has that been a point of contention at times?
Is it difficult?
Nope. But this is a mostly recent (in the last year) development in our marriage. I have always been interested in polyamory and whatnot. And my husband grew interested and curious. At the moment, it's just physical though. It took a lot of growing up on both our parts before we were willing to do this. We have boundaries and rules. And either of us are allowed to back out of the agreement at any point. If we flirt with people, or plan to have sex, or do anything, we tell each other, talk about it, discuss what our concerns are. And the people we do it with have full knowledge that we're married, our boundaries, etc. I don't recommend it for people who struggle with jealousy, emotional insecurity, communication or have difficulty being open with their partner(s), spouse, or whatever.