i think even if there were an option, not everyone would fill it out

i'm on other sites that ask that specific question and more often than not, the common choice is to pick "prefer not to say" or something similar

i think as delia says, you may just politely pm and ask if they are or if that seems to bold, maybe to say "hope i'm not offending you by flirting" ??

sometimes, even if they aren't bi or bi-curious, flirting is still fun *winks*