My ex-boyfriend is getting married, and I didn't reply to his message. When he met me, he asked me why I didn't reply to his message. I had a very tense conversation with him.

You loved him, so you don't have to hide it. Even if you hide it, you still loved him and had such youth and experience. Even if communicating with him shows that you still seem to care about him, what does it matter? Even if you care, you won't be together again. So let go of your emotions and don't have too much formality and pressure.