Do you ever feel that there might be a situation for you where you'd consider your partner to be "cheating"
Instead of cheating, I would rather frame this as the more general issue of betraying your trust/taking advantage of you. Everybody has limits obviously. If you're just pissed that your partner had sex with somebody else even though you had "agreed" to "exclusivity", then that's cheating. There's obviously no excuse for knowingly betraying your partner's trust. I would argue, however, that the exclusivity requirement is stupid and extremely selfish to begin with, so it's not like I cannot understand people that decide to cheat.
If your partner, on the other hand, is just using you/taking advantage of you - then that is not so much about sex/romance even - it's just best to separate yourself from such a person from purely rational considerations (regardless of whether they are cheating).
I quite liked the perspective of the Japanese people in the interview: they actually understood, that if their partner is paying for sex and/or if it's just a physical thing it shouldn't really matter. And this makes perfect sense - why would you deny your partner something that they enjoy? Unless you're just that insecure or possessive of your partner. Or you want to use intimacy to control him/her (that falls under the taking advantage/using category btw). Seems to be a big issue in the west.
Would you ever cheat on your partner without telling them or making it clear that it's a possibility?
I would like to believe I wouldn't. I am not that deluded to believe that I know myself that well however, let alone myself in several months/years. It may well be that I betray a partner in the future even if I find it unthinkable currently.
I just felt that the way the MC handled the situation was very inconsiderate to the girl, and felt like it wasn't properly explained away
Certainly the MC has nothing to be proud of in regards to this - but I believe you would agree he was not in a normal state of mind. The reason his gf just let it go can be for many reasons, so it's not necessarily a plot hole. If you're looking for plot hokes - that manga has bigger issues honestly.