Sports bras, bandeus, and bralettes designed for fashion purposes are typically acceptable - although not everyone will agree on that so you might get some inappropriate comments from people incapable of minding their own business. For a general rule of thumb, the more closely it resembles actual lingerie the more likely it is to simply be mistaken for it. Just use your head on that one. Pairing with a lightweight kimono will keep you cool and generally look a tad classier (if that's something you care about).