Whether or not someone is willing to cheat actually has less to do with their sign than who they are as a person, but each sign does have specific traits that might make them more likely to cheat than others. Aries are only likely to cheat if they get cheated on first. If they get lonely, they will seek out attention elsewhere. Otherwise they are known for being very loyal. You have to watch out with Geminis. They are the sign more likely to cheat on an emotional level, and that emotional level can be more dangerous than anything physical because they absolutely crave a mental bond with others.
No matter what stage your relationship is in, cheating is absolutely devastating. Once it's happened to you, it can be incredibly hard to get over, and even more challenging to learn to trust people again. It's hard not to wonder what could cause a person to betray their partner in such a way. It's also natural to want to understand what makes a cheater tick, so you can avoid winding up with one again in the future.
Cheating is something that happens a lot more often than we want it to. Those who cheat for some reason feel like they have reason to do so and those who do not are forced to feel terrible. Whether or not you think you have a good reason or not there is no excuse that can make it okay. Cheating is wrong no matter how you look at it.