Why would a guy pretend he’s not seeing anyone when talking to his ex?

Just thoughts. I got together with my ex and we hooked up, I told him about my one night stands and my last break up, he said he didn’t see anyone since we broke up, not even a one night stand (actually he slept with one of my acquaintances and he doesn’t know I know) Why would he lie?
Why would a guy pretend he’s not seeing anyone when talking to his ex?
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