it's probably a preference thing. one identifies as a straight person but likes having sex with whomever they choose. also, it may be an occurance thing. one is willing to have sex with any gender but tends to or has only had sex with members of the opposite sexual phenotype.
now everyone knows that i think it's silly to identify by your sexuality, but that's the way it goes. i'm not gay, i'm not straight, i have sex with a [sirgrunwald]. he happens to be a man. i'm not with him because he's a man, i'm with him because he's himself.
but think about it this way. i know a woman who identifies as masculine but not necessarily as a man and only dates men. what would you call that? i know a man who identifies as feminine and maybe as a woman but not in the way that he'd necessarily change his physical gender, and only dates men. i had a friend in college who was a man and identified as male and was attracted to men but only dated lesbians. that's just how it worked out. i'm female, and i identify as a feminine me (not necessarily as a girl, really and not even as my name, i never introduce myself because i don't think of myself as being my name), i have only dated men, but i have dabbled with women and been in love with one. what do you think that makes me (besides the gay man you thought i was before)?
we label people to make them easier to understand. we do the same thing with animals and it's called taxonomy. but just like there are no real defined "kinds" in animals and there are those who cross the lines we've defined, there are no real kinds in people. it's really okay to define groups of people, but you have to be willing to be flexible because people are individuals and not parts of groups.