Is monogamy dead?

I have been seeing this guy for two months. Everything was going well. We both agreed that we don’t want a relationship and was open about our past but I also told him I’m a monogamist. The other morning after we had sex and he slept over we were talking and I saw a text from someone he was talking to on hinge. At that moment our bubble burst and the tone changed. I also proceeded to ask him ever since we started dating has he slept with anyone else other than me. He told me that he slept with his ex before he left for his trip. We had a conversation about respect and his ex cheated on him because she didn’t communicate that she was wanted to stay open, but it felt he did the same to me what his ex did to him. He didn’t tell me he had sex with his ex while he had sex with me and I don’t know if it would happen again. I told him if you want to continue to see me we need to be monogamous if not I told him to continue to look for other options. He told me that he didn’t mean to hurt me and said that we should keep things platonic. I thought whatever because he said what we had was more than sex. To me it seams like I’m not enough and he wants to keep his options open.