You should not assume anything. Given any random coder, yes, the chance that they're male is larger than the chance that they are female, but the chance that they are female is still present, and if you jump to the wrong assumption too early, you end up effectively denying their existence, which is a big part of the problem.
Consider it from the point of view of the female coder: in every discussion, people will always start out assuming either that she is male or that she can't code. And it's up to her to either correct that (with all the crap that often comes with it) or not, and let people believe there are no female coders. Constantly running into people assuming you don't exist, can be very demotivating, and may lead her to look for a more welcoming job. Let's not do that anymore, okay?
Consider it from the point of view of the female coder: in every discussion, people will always start out assuming either that she is male or that she can't code. And it's up to her to either correct that (with all the crap that often comes with it) or not, and let people believe there are no female coders. Constantly running into people assuming you don't exist, can be very demotivating, and may lead her to look for a more welcoming job. Let's not do that anymore, okay?