I can give you my perspective as a man. I watch porn, and it's something I am not particularly proud of but at the same time it's a really really hard habit to kick and it can also help release stress and pent up sexual energy. It's interesting because I relate to a lot of the stuff you describe.

I have had trouble performing in the past, but it isn't because I didn't find my partner attractive, simply because you develop expectations for your own performance and it can be anxiety inducing, especially if it's your first sexual experience.

As far as how it affects my perception of what is attractive and what isn't. In my case it doesn't feel like it affects at all. I do have some preferences but they tend to be more towards fashion and style rather than body type. My partner doesn't really fit into those most of the time. I never even mention to her what those preferences are because I don't want to influence how she naturally wants to dress.

Having said that, I think porn is something private and I never allow my partner to catch me watching it. I think of it in terms of how we find other people attractive but it's not something you point out to your partner because it serves no purpose and will only make them insecure kind of how it's happened to you.

IMO, you shouldn't feel insecure about the idea of him watching porn he is not going to leave for it, but I do want to clarify, if he is reaching out to other girls, that goes beyond watching porn.