Being cheated on in a relationship absolutely sucks — there's not much more to say about that. Dating someone and giving them your time and affection, only to learn they have been double-timing you, is the worst slap in the face.
Take it from someone who was cheated on after three-and-a-half years, an apartment, and a dog — it's a trash feeling. And, one you always want to try and get some sort of revenge for.