The business they're in (wrestling, right?) has a lot of machismo associated with it. Your daughter will need to think long and hard about what it will be like long term to live in this type of world. I can see how it's exciting to her right now, but how will she feel as she gets older, has kids, etc. I also think it's very different for men than it is for women to be involved in such a world, too. This guy might not be a bad guy, but may just be highly influenced by the culture they're involved in, which dictates how relationships are, women are treated, etc. Just food for thought. I know she initially entered the world with her brother, but now that dynamic has changed, and she's with a guy that she's not (yet) related to, and that is also is a lot different than a brother-sister type relationship.
I agree a lot with Morgan, that while she can, wouldn't it be preferable for her to start her adult life learning some of the joys of being an educated, independent, free to-do-as-she-pleases woman? Why get bogged down right now in this type of experience? There's plenty of time for that to come later! I also agree that she's, as is, not really independent if he's paying for rent, food, utilities. That makes her dependent on him, and he's likely to see things a certain way because of it, especially given their unique circumstances.
I know when DH and I moved in together, we split everything, and I worked two jobs while going to school in order to keep it that way, because it set the dynamic in our relationship. It was important to me to be able to support myself, because of what I'd grown up with, and my own mother's inability to do so when she needed to (given she was a child of the Depression and grew up in a time and place where women strived primarily to be "housewives"). Obviously not everyone has this issue to grapple with, but people can still learn from others' experiences and learn to make good decisions for themselves anyway. I hate to see any person be dominating in a relationship - ideally relationships are pretty equal, mutually respectful, etc. - unless all parties agree to something different. Young women need to be smart about how they start things off. There are so many issues they have to deal with today.
I do wish you luck dealing with this issue. I know, having a daughter (and son) the same age, that it's a fine line we walk when guiding, yet promoting their independence. I just think that turning 18 doesn't suddenly make one all of a sudden mature and able to make good decisions. That's a process. I can already see the difference between 18 and 20, but there's still a ways to go, obviously. It tells me a lot that the people of your state made the law about getting married at 21. There must've been good reason for that. I'd take more of a look at it.