I'm so sorry about this, cassy! Same w/ the other posters here who are in similar circumstances.
"I know I should have not said anything, but after a fairly mentally draining relationship in the past, I don't wish to hide things from anyone."
I get this ^. But if you felt the difficult period you were going through was on a par w/ that draining relationship, maybe your D has a reason to be concerned.
Even if not, please remember that she probably recalls that relationship, too, and so is more likely to be worried if you tell her anything negative about this one.
Besides, as you tell us, she has been against all your relationships since she was a kid. So it's almost inevitable that she'll grab onto anything negative you tell her about your partner and not let go of it very easily.
Given her negative feelings about him, I don't think his talking to her will help the situation at all. Instead, I'm afraid it will fan the flames of her resentment and make things worse.
Please just remember that your relationships are not up to her. I think PPs (previous posters) have given you some good advice about what to say to her or her partner if/when either of them bring it up again. Don't engage in a conversation/argument about it. What's the expression? - "don't JADE (justify, argue, defend, explain)" any further. Keep your reply short & sweet and change the subject. If they try to change it back, end the conversation or visit. Hopefully, though, they'll respect your decision to change the subject.