Yes it exists. It just depends on the specific couple. I personally would not be able to know my boyfriend was with someone else, but some people don't mind. There are a lot of people that prefer an open relationship where they have more freedom. I get super jealous so there is no way I could handle that, but some people can. The main thing is that a couple needs to talk about what they want out of a relationship so nobody gets hurt.
The sad thing is that cheating does happen. If you find out about it or not, it will hurt a healthy relationship. If your bond is strong enough you can make it through it, it might even bring you two closer. Nobody is perfect, and it can happen, but learn from it and don't let it happen again. Trust is key!