I don't think it's justifiable.
But I think sometimes it's understandable.
I was 99.9% certain my XH was regularly using prostitutes - no actual proof just all the "I was only looking" shite they come out with for escort searches.
We stopped having sex - why would I want to? Tried to talk, tried to get him to counselling. He went once, said nothing but "I only looked" and then was "ill" the morning of the next session.
The only reason I didn't leave, was that I had a small baby. Not a financial issue, though the practicalities would have meant finding a new job. The reason I stayed so long was I didn't want to take away a family life from my child, and I didn't want to be handing her over for days at a time. I lost hours going over it in mind, crying... and living a separate sexless life.
Then an ex got in touch.
He's one reason (only a small one) that I left. He lived overseas but was coming back in a few months and I knew I would have sex with him. I had gone 5 flipping years without! And had a husband who was constantly cheating with prostitutes (I later had the evidence, he'd done it the whole time we were together)
Now I didn't have sex outside my marriage, because I didn't want that to be me. I due have text sex though... which I also consider cheating.
Should I have? No.
Was it cheating? Yes.
Do I feel remotely guilty? No.
Would anyone on here condemn me? Not many.
So I think my case is understandable, but still not justifiable.