My motivation was that I would also always prefer to know from a potential partner's partner that they were fine with me dating their partner. My thought process was that it means that everyone is on the same page, and everyone is comfortable with the situation.
I had been told that H was quite unsure of dating someone who was in a relationship already, so it seemed logical to me to try and reassure her. And I don't have a history of coming across badly when drunk.
I think you make very valid points, AM, in that he shouldn't blame me for her dropping out of contact (it's a very valid point also that she is half his age). His relationships are his own, and I can understand why he felt I was sticking my nose in, but I think (and he now can acknowledge) that he was far too harsh.
As far as monitoring this for future poly entanglements, it is a source of great worry to me that he won't be able to handle poly, as it is a dealbreaker for me. I know that there are plenty more fish in the sea, but I love this man and would like to make this work if at all possible. I am concerned however that sometimes in the interests of making things work I am more likely to apologise and accept blame/responsibility for his feelings when I should be telling him to man up.
Thank you everyone for your input.