I don't like what GP is saying either; it's sneaking some very judgmental opinions under the umbrella of "they want to force me to have sex with X", which is bad faith.
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".
The problem is with the generalization, which is present in the entire comment. I can't imagine the original conversation was significantly less abrasive.
Generalization is the problem. It is not accurate to say that all gay relationships require two penises.
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.
I can't believe I need to say this, but there are trans women with vaginas and without testes.
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.
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".