Gotta agree with
eyy_b0ss here.
That's a fairly lousy argument, especially since it's technically not NTR. It is a "romantic interest" but not at
the current time of the scene, no romantic interest has been established, etc. That's where this applies, you're not being cucked! In fact, since they're a "future LI", that means you end up cucking the other guy (which by the way,
IS STILL NTR, you're just not the victim).
The argument you use is like saying: "I'm not going to fuck any girl that isn't a virgin because she's been fucked by another man before".