Orihime can hit a lot of NTR flags:

>Lives alone, no family to protect her from shady playboys, some guy could come into her home to fuck her everyday and none of her friends would know
>Already said to be beautiful by everyone around her in the first chapters, later in the manga her body becomes increasing lewd, her tits almost double in size
>submissive and self-sacrificing personality
>She is insecure about being good enough for Ichigo
>Kidnapped by the villains in one arc, with subtle hints that they were going to corrupt her, actually had some light sexual stuff done to her, with Nnoitra shoving his fingers in her mouth
>Loly and Menoly felt jealous of Orihime because they believed Aizen was fucking her
>She never showed any resistance to mind control abilities like Book of the End, if Tsukishima had changed her past to be in love with him, neither Orihime or anyone around her would ever notice anything wrong
>She would 100% fall for Pepe "The Love" power

All that said, aside from mind control antics, I don't think Orihime would cheat after she got married to Ichigo, at that point she has solved her insecurity issues and got her dream marriage.
But I can see the younger insecure schoolgirl Orihime, depressed because Ichigo never pays attention to her, being pursued by some playboy and eventually being pressured/forced into having sex and "falling" in love after enduring months of bed breaking sex everyday.

But can you even count as NTR when Ichigo never had romantic feelings for her back then? He would just congratulate Orihime if she got married to someone else and would move on with his life.