I don't know, being angry about getting cheated on feels very justifiable. A relationship of boyfriend-girlfriend is still a commitment, however shallow or unwise it is (if you need me to explain why a marriage is a commitment, you should get help).
And Netorare inevitably turns into cheating because by definition, the girl is inevitably given a choice...and she chooses (or decides to tolerate indefinitely) the other guy for whatever contrived reason the human brain can conjure. THAT is cheating, period.
And by the same definition, Netorare is presented to us in the perspective of the guy victim. We don't self-insert, but we feel bad for the guy because, except in a few rare cases, he doesn't deserve it (having a small dick/being busy supporting the family justifies getting your girlfriend or wife fucked away? Really?). We're sad or angry or frustrated or whatever mix of the three.
Most of your statements here are wrong. Your perception of what netorare is, is off.
You don't have to have a relationship at all to have netorare. Netorare is a love interest being taken away and the emotions that arise from that. When an MC has a crush for an example and the crush starts a relationship with someone else. It's the weakest base for netorare, but it's very possible to make that heartwrenching as well. As such, netorare is not necessarily cheating.
Fact is, many of us has experienced this type of netorare to one degree or another. Imagine your twin fucking the girl you love in the next room for an instance. I'm sure it's happened, somewhere. Or when you find out your best friend liked the same person you did and one of you started dating them. The other person would be experiencing netorare, that's for sure.
Gender doesn't matter either, it never matters, only perspective and loss of a love interest does.
"The girl is inevitably given a choice"
No. There are stories where the girl never gives in, but instead has her freedom to choose taken away. It is still netorare.
If a guy borrows $100k from the Yakuza and they come to collect and he can't pay up. If they take his wife instead and use her as a captive whore to pay them back and maybe send him pictures of her in the act "to assure him she's ok"(to mock him).. That would be netorare.
The thought that you have to have a relationship to have netorare, or that it'll always result in cheating, is wrong. And because so many people think so it's hard to tag games properly, which results in the same discussions being repeated again and again.