Does "gay" necessarily mean male homosexual?

Gay can refer to men and women who are sexually attracted to people of the same sex, though lesbian only refers to women who are so.

I agree with Jasper: In Salt Lake City, Utah, it is not uncommon to hear a woman describing herself as gay. So it can be non gender specific, just like the word dude.
It is OK to use it to exclaim to a woman, "dude, you rock," but if you were to say, "dude looks like a lady," that would be gender specific.

At least in North America the term seems to be "LGBT" which does imply that they are reserving Gay for men only.

Or it could just be the natural tendancy for any 'protest' group to split into smaller and smaller sub-groups to differentiate themselves from the other identical groups (especially the people's front of Judea!)

In my experience the term has increasingly been used only for men. It wouldn't be improper to use it to refer to women, but I've not heard the word used to refer to an individual woman in the last 10 years or so. (For a group of mixed gender it's probably more acceptable.)