No Exes Rule

Would anyone here date someone who still talked to their exes, let alone hang out with them? Is this a red flag, and why? I wouldn’t do it, as I’d already done this once when my ex wasn’t over his ex, and he ended up lying and cheating multiple times. I realized I’d always be second to her because of how much he talked about and compared me to her. I eventually ended it, but he still decided to email and text me through his sister's phone after I blocked him. NEVER AGAIN.