Actually there are straight couples that are somewhat atypical as well, when it comes to the gender roles. I'm currently in one of them. (My boyfriend is not submissive, exactly, but I think I am probably *more* dominant than he is, and when we got together *I* was the aggressor, the person taking most of the initiatives, not him.)
..Attraction and gender identity seem to be somewhat immutable. Just like you identify as a woman and probably would find it difficult to identify as a man, I too identify as a woman, and always have. It's just that I also just so happen to be *attracted* to women, and I did not choose it. Just like you didn't choose to be attracted to guys either. (Or at least I assume you didn't.)
So when that is given, some people just so happen to be guys and also be attracted to other guys, etc, I think what gender roles they adopt has a lot to do with PERSONALITY. There are submissive and dominant guys out there and there are submissive and dominant gals out there. Sometimes there are clear-cut roles which are a lot like the roles in an average heterosexual relationship. Take butch-femme lesbian couples, for instance.. Often they work in approximately the same way as straight relationships, but not always.
..Then there are lesbian relationships where none of the people involved are *completely* butch or *completely* femme. I've been in two such relationships, and it works perfectly fine. What happens is, there is not just ONE person wearing the pants. It is possible to SHARE that role, if not evenly, then at least *somewhat*. Even though I was the one who started hitting on my then girlfriend, SHE just as well could have, and often, after we got together, SHE was the one taking the initiative.
..Hope that helped. Tell me if you have any questions. =)