Just keep your hands up at all times and you'll be fine.

Seriously, though, if it's somebody's occupation, why not? I'd be more worried about dating a butcher to be honest. (Except Sam from the Brady Bunch. Hands off, ladies, he's all mine...)
This is an old thread so somebody probably already raised it, but serious dating for some could be masked not by fear of what he could do to you, but the fear of the occupational hazards he might suffer himself in the squared circle.
There's always the trepidation that your partner might come home one day bloodied and bandaged, eyes the size of small grapefruits, his nose relocated toward one side of his face, wheezing from his tenderized ribs, maybe not at all clear about where he was last Friday, or where he is now, maybe needing some assistance to suck soup through a straw until his jaw gets re-wired, mumbling incoherently like Leon Spinks, insisting on a rematch. And I'm sorry, but being the gay Talia Shire is just not what many guys feel cut out to be.
I'd imagine it takes a person who's into a boxer for EVERYTHING OTHER THAN the physique and the pugilism, because once said boxer's career is through, the partner may have a completely different physical specimen in his arms.