Ok, so after reading this thread and giving a bit of thought, I may have some reasoning as to why you women are the ones who wear the costumes. First and foremost, women have far more intricate fantasies than men do. The fantasy scenarios women create include very specific settings, even the time of day is included in these fantasies, there is, at times, even a rough dialogue women have in their heads for these fantasies. So simply dressing up in a costume is a mere half-ass attempt at creating these fantasy scenarios, the attempts end up being funny rather than sexy. Men's fantasies are much, much simpler; we want a beautiful woman who is more or less open for some sexual experimentation. A cute costume, some nice lingerie, we see it, we see how sexy the woman looks in it and we immediately are ready to rip whatever clothes she has on right off and get to the sex. This is really just more evidence that a women is more turned on by sensations and emotions, men well we are simply turned on by seeing a bit of skin.
Someone else said it earlier, but blow-jobs are like flowers for men. We really don't want little gifts or cards, we know our women love us because they give us guys sex. Women would prefer to be surprised by a piece of jewelry, flowers, or any little gift that "proves" we have been thinking about you all day. So again, dressing up in the sexy costume means we are getting sex and therefore the only thing we need to see to know that our women have been thinking about us.
There is also the litany of double standards between the sexes. Women still expect to be taken out to dinner or given gifts, a special gesture. Men simply expect to take a woman out and have to pay for the evening. All we want in return is some sex, or even a blow-job. So to show that you women are willing to go the extra mile is not measured by how often you take us to dinner or what trinkets you by us, but by the cute outfit you put on to get us hot. (Sorry if this part sounds a bit sexist, I really don't mean it to be, but more often than not it is the truth.)
Castout, happy birthday.
Think for yourselves and let others enjoy the privilege to do so too. -Voltaire, Essay on Tolerance
There are some that only employ words for the purpose of disguising their thoughts. -Voltaire, Dialogue, XIV, "Le Chapon et la Poularde" (1766)
We must cultivate our own garden. When man was put in the garden of Eden he was put there so that he should work, which proves that man was not born to rest. -Voltaire