I'm not sure this question is really the deepest of Deep Thoughts. But fwiw:
When I first read what she wrote I didn't actually understand what she meant and had to re-read. Her boyfriend took it as him not being the sort of guy she'd use for casual sex - i.e. he lacked the attributes that she would want for that - but that he had qualities she would want in a husband. She meant to emphasise that his qualities made him marriage-worthy but she was very clumsy in expressing this. If she did mean the former, consciously or otherwise, he's probably right to be a little put out. If she meant the latter then she needs to clarify something like this:
"What you need in a casual sex partner is someone you're not too attached to, not too involved with, who isn't clingy or going to get too involved with you either. Basically, a casual sex partner needs to be a 'good enough but don't really care about them' person. I can't be that with you, could never be involved with you and keep it casual. You're the sort of person I want to marry."
That would be much better, though a little concerning about the attitude to casual sex.
If she wants to understand how she might have hurt his feelings by something that would create an equivalent feeling in a woman, she could imagine a guy saying something to her like "I'm glad you're not too hot because I feel more comfortable that you're in my league." No woman would like, nor should like, hearing that.
To the people saying this must be false and using things like "numeric username with one post", I say that it is entirely normal that someone with an identifiable or known username would make a new one just for asking this question, it is entirely believable that a Reddit user might actual do something like this irl and it is utterly plausible that the sort of person who would do this irl would also be the sort of person who would turn to Reddit for advice.
