This topic occasionally make the rounds at mumsnet and I think it worth continuing to reengage in it. There was a time not long ago when mentioning, on mumsnet, the fact that most trans women were heterosexual men whose interest in presenting as women had a sexual dimension could only be done obliquely. That in itself made the accusation taboo.
But it is instead a statement of long known fact fact that doesn't necessarily imply a value judgement or call into question the legitimacy of trans identification. We're not surprised that homosexual men and women are only sexually attracted to the same sex. The fact that there's a strong sexual component in gay and lesbian identification doesn't make it less valid. We don't think that because many find the thought of sexual attraction between two people of the same sex unpleasant, that gays and lesbians should be proscribed to be valid reasoning.
The same principle applies to trans identified individuals. Characterising trans identification as a greedy pursuit of unlimited (and unnatural) sexual pleasure is a value judgement and one I would personally dispute as a blanket statement. But acknowledging that for a large number of trans people, especially heterosexual men, presentation as women is the only way they feel they can exist as sexual beings - and humans are sexual beings.
The big difference between homosexuality and trans identification, is that the argument that homosexual relationships are really no different than heterosexual relationship (just same sex) is an attractive one, and the tendency is to apply it to trans individuals: trans women are no different than actual women. That is of course false, trans women are in no sense real women. But it is true that they are not that much different than the average non-trans identified male (because they are men) - that doesn't make them morally objectionable.
And it should be noted that being 'for gay rights' doesn't mean that you must think that gay relationships are exactly like straight relationships (though many do believe that) - those rights stem from principles of autonomy, liberty and privacy not equivalence to heterosexual norms.