The OP is apples and oranges, comparing lesbian/gay with trans. The first is a sexual attraction/orientation, the second is a so-called 'gender identity'.
Flora said "Born this way" seems to me to be a way to explain to conservatives why they should accept gay people. and to that I agree. What the TRAs did, was hijack that popular 'sound bite' to gain acceptance on the same terms (ie, can't help it, gotta accept them).
As far as sexual attraction goes, there is enormous societal pressure (from patriarchy) for females particularly, to 'be heterosexual'. That brainwashing for girls starts early. This is what sets off my 'spidey senses', as to why patriarchy would put so much damned effort into convincing girls that their Life Mission is to find a man and have babies. It tells me that, most likely, if women were left to their own devices, they probably would not engage in heterosexual relationships (but would perhaps briefly, for the purposes of reproduction).
There are some lesbian/gays that just know, right from puberty, that they are ONLY attracted to same sex. But a heck of a lot of lesbians find lesbian relationships after the earlier het one has broken down. In the latter case, I do not find that these women are bisexual, but 'hijacked lesbians' from earlier brainwashing. And this is why I reject a hierarchy within lesbianism, or rather the purity contest ('gold star').
TL;DR Most people are probably 'more or less' bisexual, but compulsory heterosexuality forces many into heterosexual relationships, particularly early on. And some lesbian/gays have a much stronger attraction from the beginning.