Bisexuality is not transphobic because the capacity to be attracted to someone regardless of their gonads is an inherently transgender characteristic. For it to be otherwise is anathema.
That is to say, whether any transgender person finds someone of the same sex attractive they have that capacity; not everyone does. For example, a couple having one transitioning partner seldom lasts unless the other partner is either bisexual, pansexual, or transgender.
Far from bi- or pansexuality being on the spectrum between straight and gay, I'd say it is instead on the spectrum between cisgender and transgender. I would even go so far as to say that bisexual and pansexual people are what those who are a little trans look like.
Gays and lesbians realize this on a subconscious level: that bi- and pansexual people are not the same as them. This is because marginalized people define themselves by how they depart from the mainstream. Gays and lesbians depart from the norm by sexuality, not gender; whereas transgender people depart from the norm by gender, without regard to sexuality. This leads to gays and lesbians being more parts-centric and bi, pan, and transgender people being more gender-centric.
The lack of a group's departure from the norm is where any possible exclusion of bi, pan, or transgender people comes from. That cisgender straight people exclude gays, lesbians, bi, pan, and transgender people is because they are normal in terms of both gender and sexuality. That some gays and lesbians exclude trans people because they are normal in terms of their gender.
Transgender and, to a lesser extent, bisexual people make gays and lesbians uncomfortable the same way that gays and lesbians make cisgender straight people uncomfortable. Bisexual people make transgender people uncomfortable not at all. On the contrary, as a transgender woman, I am fond of saying: that bi people are my people.
For a transgender person to exclude someone on the basis of their parts would mean condoning their own exclusion, and this simply cannot be. This is why attraction without regard to one's gonads is an inherently transgender quality.
It, therefore, follows that attraction without regard to gonads defining bisexuality means that bisexual people are on the trans spectrum; how could transgender people find themselves offensive?
By definition, bisexual people are a little trans so bisexuality cannot possibly be transphobic.