Guess I missed the party scene...

As the title suggests, I don't really go to parties, therefore most of the experience of this journal's topic is already extremely limited. I seriously have no clue about any of the "lustiness" of the general population of women during Halloween, so the only basis about this I have is the insistence from you all that it does. Thus, the only opinion I can form is through an amateur psychological view rather than an experiential one.I also don't have much experience with Halloween in general. Living in inner-city Indianapolis, trick or treating door to door was not a safe proposition, so I have never been.
The psychology behind most Halloween costumes is actually relatively simple. Though comfort, availability, and funds factor into a costume choice, underlying factors can be easily interpreted. When you choose a costume, it gives you the ability to live out a fantasy and show others some part of your hidden self. Put into black and white terms, Halloween is a chance for people to dress up for one of two reasons. People would dress up as something or someone that they love, and wish to emulate, or they dress up as something or someone they hate and wish to mock.
This applies directly to the licentious nature of many costumes found in this type of environment. These can represent the individual's repressed sexuality, or they can be a healthy expression of someone who is not very repressed.
The concept of acting like someone else without repercussion is found within this. Feeling sexy is a confidence boost for anyone, even though it will only be for one night. Women can dress anyway they want and not be thought of as a whore.
Males could express this through different ways than sexuality, such as power. Such costumes as superheros could represent a repressed insecurity.