I find the idea of a t-shirt saying "not currently on a date" either side vastly amusing
Speaking from the female side of this equation - I would just ignore it, far as I can see there's nothing you can do to make it obvious you're not on a date that will not risk offending the person that you're out with.
Maybe it says alot about your other acquaintances that they're unable to see you in the company of a woman without assuming you're trying to get laid. Either that or you're just such a stud they find the concept of a woman being around you and not being sucked in by your animal magnetism hard to understand.
dlish it really IS possible to have a gender opposite friend who doesn't want to sleep with you - my best friend is male and knows more about me then almost anyone else (barring my SO) he's one of the few guys that I feel absolutely at peace around. There's no sexual tension there at all,
"we're too close it would be like dating a sibling"
his words not mine.