PigletJohn - the single biggest thing that can be done to prevent rape is to ensure that there is a culture of absolute revulsion about it - so that everyone knows it is wrong to have sex with a woman without her freely-given consent, wrong to have sex with a woman who is incapable of giving consent because she is under the influence of alchohol or drugs, to give a woman alchohol or drugs in order to render her incapable so that you can have sex with her.
This t-shirt does the exact opposite of that. It feeds into a culture that thinks rape is a joke, funny, something to be taken lightly. It gives the impression that maybe having sex with a woman whom you've drugged with rohypnol isn't really rape. The t-shirt shows a huge lack of respect for women and for their right to respect and to decide who they want to have sex with. On its own, this t-shirt isn't going to make a man into a rapist - but if enough of these 'jokes' are allowed into the public arena, and are not challenged, then it will encourage a culture of 'rape is OK, rape is no big deal'.
Do you really want your mother, sister, wife, daughter, female friends walking around in a culture that thinks rape is something funny or lighthearted? Do you want to run the risk of them meeting a man who has taken on board the message that rape isn't really that serious, that it's OK to sexually assault a woman or rape her or drug her?