Two factors I haven't seen addressed here:
1. There remains an enormous stigma on being bisexual in both the lesbian and gay communities.
Most lesbians and gays categorically refuse to date or have relationships with people they consider bisexual. They regard bisexuals as promiscuous, lying, deceiving heart breakers who are selfish in bed and will leave you when they decide to return to their boyfriend or girlfriend, or just go back to their two timing slutty bi ways.
I wish I was kidding. So there is a huge temptation to want to be in a relationship with a trans person but still avoid the bi stigma by any means necessary.
2. Romantic orientation and sexual orientation are not always the same. I have had plenty of gay guys fall for me, but they can't deal with the fact I intend to have bottom surgery. Many MTFs are in relationships with gay men when they come out as trans. Some stay together but most don't make it, often because the physical attraction stops with physical transition.
3. Both romantic and physical attraction depend on experience. They are not abstract. People decide what to call their orientation after they have been exposed to potential attractions, and usually trial relationships, and become confident in the pattern. Most people have not known trans men and trans women as potential partners and won't know if they are attracted romantically or sexually until they are.
4. It is very common for people's horizons to expand after being close enough to a trans person who is their type romantically and/or sexually. If we deny the possibility of lesbians or gays (in the sense of almost never attracted to opposite sex cisgender people) being attracted to the opposite sex transgender individual, then we also negate the experience of straight people (in the sense of almost never being attracted to same sex cisgender people) who find themselves attracted to same sex trabsgender individuals. And we may negate the experiences of trans* people who discover they are also trans-only bisexual.
There isn't a word for trans-only bisexual, but maybe there should be.
I dated and almost married a trans man this year. He was the most essentially male person I have ever known. I would absolutely consider spending my life with a man who is trans.
Being with him was the catalyst for going from "i might be bi" to "i know i am bi".
But I understand why a woman could love a trans man while seeing him as a man, even though she can't stand straight cis guys, and so would keep calling herself a lesbian to avoid her inner biphobia and external biphobia.