Often a guy will think that because a woman is wearing sexy panties, she must want to have sex. It may even be used as a defense for rape: "but your honor, why else would she be wearing black, lacy underwear? clearly she was horny." If a woman's undergarments are used as a basis of assumed consent, could they also be effective in communicating non-consent?
Enter the What Would Your Mother Do? conversation underwear that communicates that sex isn't happening that night. The line is supposed to reinforce abstinence among teens, but the actual panties don't say anything about sex being shameful. Sure, they may be a bit too flirtatious for my taste, though I kind of like the one that says "not tonight;" I might go for ThinkGeek's "403 Forbidden" panties if I wasn't in the mood that night. I may also wear a pair of simple, black panties with a message such as "private property: no trespassing" or "authorized personnel only" as a way to communicate that you need an invitation to take those off, and if you do it without my consent I will defend myself.
I have no problem with the availability of panties that communicate what you're (not) up for on a certain night. Surely, it's harder to withdraw consent after it's given than it is to say no and then change one's mind. However, the latter is problematic if it always happens - I do wish women would stick to their guns a little better when it came to saying "no," and not giving in every time a guy argues his way into getting some. It's like giving a child what they want every time they throw a tantrum, it spoils them. Of course, guys shouldn't be "throwing tantrums" in the first place.
To clarify, items of clothing that say "I'm not opening my legs tonight" don't necessarily imply that the wearer is abstaining until marriage, or because sex is bad and she never wants it ever. But sometimes when you get dressed up to go out, you know you don't want to have sex that night, maybe because you're not into casual sex or you and your partner just aren't at that level yet and you want to wait. And that's fine! I don't see panties like this as enforcing a binary that either you never have sex because it's icky or you have it all the time because it's not icky. I see these as simply communicating "it's not happening tonight."