A little bit of both. Honestly. Why? you ask do I say both. Well it totally depends.
1. Was the person sincerely honestly upset that they made the "mistake." You can typically tell if it's a bs drama fest.
2. Was it planned or did it just "happen?"
3. If the person is sincerly honest yes mistakes "can and WILL happen" in realationships. Am I going to hate on my girlfriend for it no. I've made mistakes I'm sure, would I make a habbit of her just making mistake after mistake. No. That's not right either.
That's just my .02.
Although I am a open realtionship type of person anyhow. So if my girlfriend wanted to do stuff with somebody all she'd have to do is say so. I'm not so stingent. Although I am very carefull "protection wise" because of that ;).
So personally, in my realationships. I don't typically have that problem. Cause it's pretty well known about me, that if they want to all they have to do is say so. I'm not going to get upset. Or anything at all.
Although, if perhaps they didn't say something, and went behind my back, and snuck to do it. That's crossing the line. That's cheating. If I'm knowledgeable and I have a reservation where I can say no I don't agree to it. Then that's fine. As long as both partners agree it's not cheating. If one doesn't care for it you shouldn't do it at all.