Why are you specifying gays (and specifically lesbians)? I bet you're dressed "like a man" at the moment - isn't that following a gender role too?
I think the fundamental (and all too common) issue here is starting from a point of treating homosexuals as different or special and expecting them to obey different rules and instincts to "normal people". The fact is that homosexuals are "normal people". The all have a particular feature that is different from the majority but we all have features that differ from the majority. Different is normal. A truly average person would appear very strange to us.
So, some lesbians dress and act in a traditionally male fashion because that works for them. The combination of their individual psychology and the society they're in create that feature. That's fundamentally no different to the reason you (presumably) and I dress and act in a traditionally male fashion too. The factors are obviously different but the basic process is the same.
I focused on the lesbians because for obvious reasons this situation involved a lesbian couple. But expanding on this I have observed a lot of gay couples especially while travelling and partying in Hollywood where this partixular dress/attitude is common. I understand that gays are "normal people" but that is not the question here. The question is why do gays male and female pursue to have behavior of the opposite sex when they do not sexually desire, emotionally desire, spiritually desire the opposite sex?
Clothing, buzz cuts, walking, talking, even growing facial hair are a few examples. This isn't some gender identity disorder this is pure gender role fulfillment. I've even heard of a lesbian at my job who is considered "butch" using the men's restroom. Again this is beyond what makes them feel comfortable.
I just dont see how two women one being the "girly girl" and the other the "man" model their roles after heterosexual couples. Hence is why I would assune that long decades of partiarchal influence upon family has been engrained in the minds of people thus you have what I just spoke about.
I mean since a lesbian woman doesn't desire a man why actively go out of your way to look like one then get upset when someone calls you sir?