With my ex, I had an agreement to "share", and since I'm bi and he's straight, this meant only female partners who were interested in both of us.
A few years into our marriage, he mentioned to me that a close male friend had told him that "Dude, your wife is hot, I'd like to bonk her if I got the chance.", and that he'd replied "I'm only into pie-eating contests, not sausage fests." I tried to indicate that I was interested, but he wasn't willing to even consider it, and continued pushing me to look for hot bi babes we could both date.
Over time, I gradually grew even closer to the guy friend, and we talked about sex a lot, flirted, but nothing could happen while I was married. I resented that restriction, especially since my ex would flirt with girls online in hopes of "getting us a girlfriend" but then would get jealous of my guy friend, insult him, etc. The whole experience made it clear that my husband only cared about what would benefit him, that he didn't give a flying rat's ass about my happiness, that he saw me as his possession and wanted to control me.