It sounds like what's bothering you is not so much the fact that they play roughly, but that it makes her look like she's survived a dog fight, which is understandable because we're talking about 2 pitts. Pitts have a bad rap; they are wonderful dogs. The great thing about Pitts is that, as a breed, they are highly driven to please their owner(s), so both of them care what you think.
If he stops when she yips, then he knows about playing 'appropriately' with another dog. The fact that she doesn't yip often means that she enjoys the rough play.
My suggestion is to do some 'negative' reinforcement. I'm not advocating any type of physical discipline, what I mean is giving a negative consequence - a timeout - when the play gets too rough. Designate a timeout area in your home, some place where you can stick the dog for a minute or two and leave it there. When the play starts to get too rough, tell them to calm down or and if it continues, wordlessly, calmly take the male dog and put him in timeout. Have him stay there for a few minutes. Then wordlessly go get him and bring him out of timeout. If the rough play starts again, repeat. He'll quickly figure out that getting too rough means no more play and doing something boring. She will figure out getting too rough means her play companion gets taken away. Dogs are primarily non-verbal so they don't need a lot of talking. Being calm (and quiet) about it helps the dog to get it faster than if you're upset.
If your female is the one who starts it, put her in time out. This should do the trick, but the key is consistency. Don't let a time go because you don't feel like dealing with it just then.
I've fostered and rehabbed many dogs and this technique works. I originally saw it on 'It's Me or the Dog,' a program featuring dog trainer, Victoria Stillwell.