I think ntr is kind of like strapon sex and femdom. Technically, any man without prostatitis can enjoy prostate stimulation because it's designed to make us feel good and orgasm. But most men never even try this because it goes against their self-image. I'm the same, btw. And self-image is important enough for me to not even consider actually becoming cuckold to any of my GFs or forgive cheating, despite the fact that I enjoy ntr. And I have no plans to give my ass to a girl but I can consider prostate masturbation, mostly for health reasons. People in general don't want to feel weak and inferior and it's twice as true for men. They don't want to be looked down on by other people or to feel less manly even if no one else knows. And it holds us back from some fetishes by default. For better or worse, though, I didn't even think about my "self-image" while playing Kabe no Tsuma as I just liked the art and animated scenes. But I found the situation hot and exciting and this was how I got into ntr, lol. Some people are just more open-minded when it comes to masturbating alone without anyone else knowing. Others are too afraid that other people find out to even try something in secret.
Other than what you said about jealousy I think the main appeal of netorare is that you can see a woman from both sides at the same time. From one side you can see her idealized version from the beta protagonist's point of view. From another, you can see her as a slut existing just to please the alpha netori guy. Pure girls make us men feel emotionally attracted and slutty submissive girls make us feel erotically excited, but it's hard to achieve both at the same time. If a girl is slutty we become less interested in her romantically, if a girl is too perfect it makes it harder for us to want to have wild animalistic sex with her (I can barely get hard on pure vanilla sex in story/romance focused visual novels (like Baldr Sky, FSN, Muv Luv) if I'm too attached to a heroine, this almost feels blasphemous). But in ntr you get a situation where one man's angel is another man's slut, which gets the best of two worlds. One's guy perfect gf/wife is someone else's little bitch, what can be hotter? This makes both netori and netorare hot, though this approach is less common in netori.
