Well, the thing that is appealing about penises is, like, interacting with them. This is also true of vulvas and breasts and so on, but that is a subset of their appeal; they are also aesthetically attractive in a way penises aren't (obviously talking from my own perspective). I'm not gonna say there can't be such a thing as a good-looking penis, but... ime they are just somewhere in the range between "eh, it exists" and "actively weird-looking". But they allow you to do things that are enjoyable, and so my mind focuses less on the aesthetic aspects of the thing-in-itself and more on the experience they enable. In that sense I can have positive associations with other people's genitals I just can't with my own.

(I can have positive experiences involving my own genitals, but dysphoria outweighs this)