Boyfriend problems not wanting to promise not to cheat?

I've been dating someone for 4 months who I suspected of cheating on me, but I couldn't prove it. I presented my feelings to him. He said that he didn't cheat. I said I guess I just want you to commit to me. Promise me that you would never cheat on me. I said I promise to you that I would never do anything to hurt you. He said, "I cannot promise that to you, because everybody makes a mistake." I said what if we got married like we planned to in a year. He said, "Well, I will commit to you then." What do you think about his response. It just makes me even more suspicious of him.