I guess the thing is, you are unlikely to know that the radiographer is a lesbian, so there is not that immediate discomfort that you might have with a male viewing you in that way.
Personally, although I'm fine with male gynaecologists, I would prefer not to have a male carrying out a mammogram. Maybe because with gynaecology, there is often more instrumentation involved, whereas a mammagram just feels a lot more intimate, given the amount of handling involved. Having said that, on 2 out of the 3 occasions I have had one, I have been a bit unhappy with how rough the women have been with my breasts in trying to position them on the machine! Maybe I've just been unlucky.