Not necessarily a wife, but a girl that the main character is supposed to care and hope for a intimate relationship.
But yeah, NTR doesn't mean "another male exist in this world".
Right now there is no NTR content.
Actually, the definition says "wife", but I will go as far as include steady girlfriend. Never a girl you met three days ago, or someone you're just hoping to bone.
NTR is netorare, which is the exact same as "cuckold".
It also means that not only does your significant other see other men, she has sex with them. In some cases you're made to watch it (usually if your significant other is dominant).
So yeah, you're right, no NTR in this game, as it would mean that 1) you watch it, and 2) one of the females is your steady girlfriend or wife, neither of which is true.