We get into mfm with a guy we know, been doing it about 4 years off and on with him. It's not cuck at all. We take turns doing her. Nothing weird, nothing degrading, just a LOT of sex. He's way bigger than I am, he's 8+ easy, I'm 6+. She's never said anything about our dick size differences, if he's better than me or anything like that. It's VERY obvious she likes sex with him a LOT, she cums with him way different than with me, but it's all good. We don't have any "rules", they fuck in every way you can imagine, he does her butt hole most nights. We mostly do this in our bed but we've fucked her in the living room and kitchen too.
Neither of us is more dominant, one of us does her till we either cum or just want a short break. It's a nice relaxing time with her getting fucked all night.