I think there are very important differences between A) "I don't want to have kids with you" (which LW's boyfriend has every right to say, and so does anyone else), B) "I think you need to get your shit together a lot more before considering parenting" (which is iffier but still potentially reasonable from a close friend or partner), and C) "People like you are unfit to parent and should never have children."

What the commenter said, and LW's boyfriend agreed with, was C. That's not about his personal future, or LW's, or the difficulties they may or may not decide to face together. It is, as LW and Prudence noted, dehumanizing. Getting hit by that from a partner is a profound betrayal, and casts doubt on whether he considers LW an equal human being worthy of respect at all.

(And if he really cannot look at LW and see anything but a source of trauma and pain, then he should be initiating a breakup and getting some PTSD therapy, which I suspect he really needs. That's not an insult to him - I have PTSD and have experienced thinking about a traumatic incident all the time and it sucks a lot. If he were the one writing the letter, my advice would be to get the heck away from as many triggers as possible and get a therapist with some experience in treating trauma, so he doesn't have to stay stuck in that loop.)