Starting off, this is a good question, and something I've never considered...
I think the the problem is ignored for a few different reasons. The first being, I don't think it even crosses people's minds that gay people could suffer domestic violence problems too. I for one have never even heard of it happening, and it's never even once crossed my mind.
Second, typically, in straight relationships, the man is abusing the woman. It's generally thought that there is no reason for a man to ever hit a woman, because of the "unfair advantage". Of course beating anyone is wrong, but I'm trying to stay simple here. As far a same sex relationships go, two men fighting seems... Ok. There's no "unfair advantage" like with a man and a woman. People generally think, "Men fight all the time, it's ok." or that "Men usually work out their problems with themselves much better than with women." As far as two women in a relationship go, again, people don't picture it happening. (And if they do, they picture it in a much different way than what's being discussed here...)
In general I don't think anyone pictures gay couples having problems that severe. I think the straight population (I'm going to include myself in this too.) think gay couples are happier in their relationships then straight people, and would never have such a problem. It's hard to explain, but I think people generally think to do something that "out of the norm" or "frowned upon", to be in a relationship with someone of the same sex, it would have to make you insanely happy.
Does that make any sense?
I could be very wrong, but that's just my two cents.