F16 dating M16 needs to explain whats wrong with talking to his ex

okay so basically we were having a hypothetical conversation about if his ex girlfriend needed somewhere to stay and had nowhere else could she stay at his

i said no because she shouldnt even have contact with him and he said 'why?'

i feel like its just like basic to know you shouldnt talk to your ex but when he asked why i dont really know how to explain what the issue is exactly

so can any girls help me out with explaining because all i have so far is 'it just feels wrong' and no logical explanation even though i trust him and know he would never cheat i just do not want him to talk to an ex girlfriend of his