Far from being offensive, or an admission of dubious morals, I think that recognition that you can never say with certainty indicates a higher level of maturity and emotional intelligence than all the hard no's, and actually serves as a protective factor.
To view yourself as completely immune to a behaviour/situation is actually quite risky as it can lead to a failure to recognise danger signs and moderate your behaviour. Many affairs develop from emotional affairs, which in turn develop from close friends, which have resulted for getting on just that bit better with the hilarious supportive person at work who brings a bit of spark into the humdrum. People who understand human fallibility are more likely to see where that can end than those who insist that these things are risk free, because I trust them/ we'd NEVER cheat.
I think, as quite often happens on the internet, people are saying broadly similar things but coming at it from different directions and then disagreeing with each other. For me, part of having a moral code, with includes fidelity, is precisely that continual checking and being on guard. It's just being aware of my own reactions to things. Both my parents have either cheated on each other or tried to cheat but failed. When I was a teenager my next door neighbour left his wife for another woman with whom he'd been having an affair for years. There were cries of 'not Jim, he isn't the type!' And I remember as a teen thinking well, clearly he is the type, because he just did.
So the reason I'm as certain as I can be that I wouldn't cheat is because I do guard against it and keep questioning myself, because I've seen how common cheating is. I know that at the moment I feel revulsed by the idea of being intimate with anyone but my DP but I'm also aware that that might change in ten years time. And so I'll be on the lookout for signs of that. I don't mean it's at the forefront of my mind. It's just an ongoing thread running along, quietly.
I also think the idea of a type who cheats is a bit ridiculous. It's as if people have this cartoonish idea of a flirty, cheeky, outgoing type who is more likely to be unfaithful. The reality is that quiet, unassuming men like my dad or my neighbour can cheat given the opportunity, if they lack introspection or are too cowardly to end a relationship before starting another one. But again, that doesn't mean anyone can cheat. It just means people need to broaden their idea of who might.