Someone ask you if you have a girlfriend and you feel that you are not being yourself if you do not go into an explanation that no you dont have one because you are gay.
Whats wrong with just saying you dont have a girlfriend?
Its almost like a defense over some identity.
That's a good question and the whole discussion of societal norms is a fascinating read in good books on anthropology. "Does he have a girlfriend", "does he have a job", "did he go to college", and so on are all more or less pecking-order questions that people use to sort out who stands where in human society, who is desireable as a mate, who gets what favors because they can return favors, and so on. If you're really interested in the topic, it's worth an extensive read.