TWO, the armpit is a sensitive area, either for men or women. “Sensitive” means ticklish and easily to be hurt. So it’s nothing to do with gender factors. Even a muscle and macho man can not stand when his armpit is attacked.
THREE, you don’t feel ticklish when you’re tickling your own armpits. According to Aristotle it is because you have anticipated and have a full control of the tickler’s fingers i.e. (your own fingers) movements. But when it is your partner’s fingers that tickle you, you’ll be paranoid to anticipate them, so that laughter arises.
FOUR, the woman generally loves when a man admires her armpits. But not the other way! Woman tends to look away the view of man’s armpit. I don’t know whether this is a question of culture (woman never obviously shows the interest in direct) or woman is not attracted to man’s armpit indeed. But…
FIVE, there are Indonesian artists who had publicly declared how much she has enjoyed male armpit (in this case her own husband), anyway. For instance, Titi DJ and Chintami Atmanegara.