...and proud of it? Or just want the nearby men to know?

Basically, I think the shirt is a subtle and clever way to tell the world "I'm a slut." Which could be a critique of our society's double standard regarding gender and sexual freedom. And it could be a way to let all the men in the vicinity know you're an easy mark. When I see such t-shirts on women, I tend to assume the latter. Mainly because I usually see them on inebriated females, hanging onto the nearest man for support.

Such women are challenging the norms in their behavior if not in their ideals. If we wanted to challenge the double standard, we would not look down on these women for their sluttiness. Rampant casual sex would be simply another loved behavior, like rampant casual eating of the potato chips.