Because double standards. It's always ok for the guy to get as much pussy as he wants, but heaven forbid if a girl so much as looks at another dick.
Basically, NTR used to actually mean a specific fetish with a specific theme. Nowadays, any time some kind of molestation, not even full blown sex, happens outside of the 'primary' relationship, it's considered some kind of NTR. There's probably a dozen "types" of NTR nowadays, including one where the kid's mother fucks a new guy instead of her own kid who obviously wants to dick a milf, especially if it's his improbably hot mom.
Also note that this almost always pops up when female characters are promiscuous, willingly or otherwise, and almost never if it's the male character that fucks around outside of their primary relationship. Because again, double standards.