IADad said:
your house, your rules, but I think it's a dangerous precedent and going to cause youtrouble if you keep having vastly different rules, at mom's or dad's house. I think you need to come to an understanding with her, about why it's okay and in what context he's watching, i.e. you're there to explain the sport of it etc., and that they are trained professionals etc. i think a certain amount of context is important.
Ditto to this and to the comments above about picking your battles.
As far as MMA goes, it IS sport, but there's no question it's a very violent sport. The times I've watched it, I've seen guys beaten bloody by repeated forearms to the head. It's far, far more violent than hockey, in which there's an occasional fight these days, most of which are nothing more than a few swings and misses before the refs break it up. I don't know how old your kids are, but I wouldn't expose children to MMA until they are old enough to understand more about it. Absolutely, kids will try to imitate the things they enjoy.
When kids enjoy watching hockey, they will want to go out and play hockey, not fight. When kids enjoy MMA, they will want to go out and fight their friends (or siblings).
Honestly, though, I think the more important issue here is this: You need need NEED to find a way to improve your interactions with the ex-. This is important. You two are the adults. If she says "I'd prefer you don't let X watch MMA," you need to be capable of NOT taking that as her trying to control you, and turning it into a pissing match. You need to be able to rationally discuss it and come to an agreement, and yes, sometimes you need to agree to let her have her way and back her up on things. As it is, it sounds like every time she says anything, you get your back up, and then you fight about it. You guys need to break that cycle. And I'll say more specifically that YOU need to break it, mainly because she's not here to talk to about it.
Your kids are, I believe, still young. As they get older, they will TEAR YOU GUYS APART if you are on different pages. You really, really don't want to be here five years from now, dealing with all of the crap where the kids tell you they hate being with you, or "Mom lets me" or things with the ex- escalate because your son is drinking because "Dad drinks all the time" or something like that. You avoid that by having consistent rules at each house and backing each other up when the kids misbehave.
Just a thought.
Good luck!
~s