No, that is how it's written in the tag rules, but that's not how the tag actually operates on this site. There's about
250 odd female protagonist cheating games that are tagged as netorare. Maybe a small fraction of which are reverse netorare where her lover is being stolen, otherwise they're pretty much all netorare where she's being stolen. That's how female protagonist cheating doujins are tagged too which is where the term actually comes from.
Just a few random examples,
Shy Girl,
Laura Lustful Secrets,
Therapy,
Curse of Kubel. Those aren't games about the protagonist being cheated on, they're games about the protagonist being stolen away from her lover and cheating on them.
As for why MMC cheating games aren't tagged with netorare, that's because Japanese is an unnecessarily gendered language. That's why a man getting raped in Japanese games is reverse rape, or why a woman's harem is a reverse harem, or why a woman's male lover being stolen is reverse NTR. Whether you think that FMC cheating games should be tagged with netorare or not isn't really relevant compared to nearly every single one of them already having the tag.