I think you have to look at the circumstances though. As I said earlier, for me personally, cheating kills the relationship the cheating is involved in, not the character, but you also have to look beyond that.
They always cheated on other people with each other. And no, that’s not a good thing, and I’m not gonna say it was the best way to go about things, but Santana was closeted and dating boys that she didn’t care about. She cheated on them with Brittany in high school, because Brittany was the only person she really did care about. None of her boyfriends were ever serious, and we saw that. Brittany cheated with Santana too, and that can pretty much be put down to the fact that, again, they’re in high school, and although she would be quite happy to be with Santana and be sexing it up with nobody else involved (proven by their being together later on with no indication of Brittany wanting anybody else), Santana isn’t ready for that. So, even though it’s wrong for her to do it, she continues living her life and dating other people, all the while continuing her thing with Santana.
They weren’t faithful in the relationships they had because they would always be with each other, even when they were with other people. When they got together and made it official, they never cheated, and neither of them showed any desire to, because they had the person they had been wanting throughout their other relationships. The way they started their relationship was messy, and they weren’t fair to anybody that they did date in high school, but their story wouldn’t be as interesting as it is without all of that mess.