The problem is with the generalization, which is present in the entire comment. I can't imagine the original conversation was significantly less abrasive.
It is also wrong to call a lesbian transphobic because she finds testicles unattractive and vaginas attractive. She is entitled to be honest about her sexuality.
Nobody says you're not allowed to have genital preferences. Chromosome preferences (with everything else equal) however seem legitimately transphobic to me. They're more a rejection of trans people than stemming from a preference in attraction.