I've cheated once before. The story is rather long, and I'm not that keen on telling it right now, but I did
write about it a few months ago in a similar topic.
Generally, I'm a very loyal person. However, when it comes to relationships, unless I feel a strong bond and that "we're truly together" sensation, I have the tendency to leave a foot in the door. That's mainly because of personal reasons and trust issues, though. It still never extends to cheating. For me, cheating is an occurrence that is born through a problematic, unbalanced, unhealthy relationship and more of a symptom than the illness itself.
I've been cheated on, too - by the very person I cheated on myself, actually. I forgave her back then, mostly because I didn't really feel the "bond" yet. We were in a long distance relationship and had been together for only a few weeks - she was drunk, and it didn't even feel like a real relationship yet. It did take away some of my trust for her, though, and I never really even expected her to stay true to me afterwards. Soon after that incident she completely wrecked the rest of my trust for her, though, and it never came back, not even when we got together for the second time. That sums up the borders and guidelines of my trust and loyalty: if we're friends, I pretty much automatically trust you and believe in you, but if you fail my trust, it will never grow back again. Ever.
Cheating obviously isn't general ISFP behavior, though. Even for me, my experiences with it were completely limited to that one relationship. Before my ex-girlfriend I couldn't even think about cheating and definitely didn't expect anyone to do that to me. It's still the same, really - although it makes me a huge hypocrite, I dislike cheaters and don't think you should be with someone in the first place if you're even thinking of betraying them. Nowadays I've just learned to prepare for everything in relationships and have a hard time giving pieces of myself to anyone, since I'm already ready for them to betray my trust. I almost expect it. That's the reason behind the recent "foot in the door" technique, too - when I'm not fully in, I can't get hurt.
There's a difference between cheating and keeping your shields up and options open. If I ever got to a true, heartfelt relationship, I simply couldn't cheat - it's an impossible thought. Same thing with a less serious thing, too, since I still am a loyal person, but in that case there'd be that lingering feeling of guarding myself and sort of preparing for the end already. It's not a good thing, since although it's not a direct letdown like cheating, it's extremely hurtful for the other party on the emotional level.
ISFPs hate having their freedom taken away, though, so to find yourself with a needy, clingy partner is like hell on earth.
This x 1000. When someone's too much of a pushover, I tend to just run away. ISFPs are like mice, it seems. Baby steps and all that - take it too far too soon and we'll disappear.