If you are for gay rights, then you accept the concept of sexual orientation. But sexual orientation presupposes a genital basis to manhood and womanhood. This directly contradicts the central premise of gender identity. Therefore, the very concept of sexual orientation is transphobic.
If you support trans rights, then you accept the concept of gender identity. That concept locates the basis for manhood and womanhood (and whatever) in the mind, making the body irrelevant to one's identity as a man or a woman (or whatever). This just makes complete nonsense out of the concept of sexual orientation and thereby erases the identities of Gays and Lesbians. Thus, support for the transgender community entails homophobia.