The joke was originally - or at least in an earlier incarnation - "My mother made me a homosexual ... ", implying a MALE homosexual, thus being a play on a popluar notion in the field of psychology of the day ('40s-'60s), that an over-indulgent mother might inadvertantly "make" her son a homosexual. I don't think this notion was taken to apply to a girl-child "becoming" a lesbian (someone can correct me if I'm wrong). So, the earlier version of the joke had a little more punch.