well "heterosexuality" and "bisexuality" are just ideas that arise from Lord English's introduction of gender to culture. since troll reproduction has no relation (presumed or real) to their idea of "gender", i think it's natural that most trolls don't even think about things like that in the first place and so it wouldn't even come up much, especially in the realm of personal identity. like you as a human might have an aesthetic preference for partners with a certain hair color, but you wouldn't use an identity label just for that. for many trolls it's probably like that.