I mentioned on May 23rd that the rate of women who've been abused in lesbian households is 35.4%, compared to only 20.4% of those living with a male partner.
Women, in general, know that if they hit a man, it probably won't cause much damage, so develop fewer inhibitions about lashing out that way.
If a woman hits a man, it's simply not taken that seriously. A woman who, say, punches a man in the arm is probably not going to be strong enough to cause any real damage, even if she hits him as hard as she can. In fact, she's likely to be laughed at.
So if she does hit the man she lives with, chances are that act won't feed into the statistics on domestic violence. (If men did report such incidents, the statistics would look much different.)
On the other hand, if a man hits a woman, that is taken much more seriously. So normal men develop inhibitions against that sort of thing.
When two lesbians set up house, they will each have a lifetime of not being seen as physical threats, and therefore don't see themselves that way. Yet, eventually, they will get on each other's nerves just like any housemates do. And, when they do, they are less likely to have inhibitions about hitting each other. And sometimes hitting can escalate.
Thus, the higher rate of domestic violence among lesbians.
That's the only theory I can come up with, anyway.