Full disclosure, I'm bisexual, currently in a male-female relationship (but last year in a male-male relationship).

1. The religious right (Xtian and otherwise) and others tend to clump people as straight vs. everything else. Hence the LGBT(etc) clumping of strange bedfellows (pun sort-of-intended). Certainly I've seen my fair share of lesbian/gay bias against bisexuals. We get blamed for everything. The common anti-(whatever) argument starts with "it ain't natural". This has gone all the way back to biblical times.

2. The raw data indicates the existence of homosexuality in nature. For a while I owned a cage of budgees who paired up in all sorts of "unnatural" relationships. One bird in particular was clearly gay. No amount of bias, personal perspective or whatever can modify the raw data. If it's the book I'm thinking of, I own a copy. 3/4 of the book is a bland statement of the data, page after numbing page of it.

3. Transgendered and gay/lesbian are quite different. I've had close friends of all persuasions. I had a trans girlfriend for awhile who was the possibly the most feminine person I've ever met. She felt towards her incompatible hardware like you would a cancerous tumour. In the animal kingdom it would be difficult to disguishish a transgendered individual from a gay or bisexual one due to communications barrier.