It's just a really shitty thing to do, and it's not even accurate. Attraction models do not solely work based on genitals, there's a lot of other characteristics that go into that.
But you don't know how his argument with his friends went, so it's a little arrogant to tell him "they were probably mad at you because you said X".
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.