It might motivate the assumption, but it doesn't have to be the case. One can imagine a model where there is some fact about a person that determines who they can fall in love with, and that hetero-, homo-, and bisexuality fall out of that for people with interest in sex, but which is still present but possibly less observable for asexual people. I have no particular confidence that this matches reality well at all, but it should be enough to show that there's no logical implication...