Why would a guy hide a new girlfriend from his ex?

Can you think of reasons, why a guy would hide his new girlfriend from his ex? Assume they see each other frequently because they live in the same community, have similar circles of friends etc. The new girl can only be there if he knows his ex will not be there. She can only visit him late at night, because there might be the chances that the ex crosses their path.
In the given case the ex does not want to have anything to do with him and they do not communicate. The relationship ended over 1 year ago. She was very clear that she does not want to be given as a reason for hiding the new girl, because it is disrespectful and not fair. Ex girlfriend assumes other reason, because she told him not to do that and that the new girl has nothing to fear from her.

What does that mean? Why would a guy do that to the new girlfriend? Girls: How would that make you feel if you were the ex or the new girl?
Why would a guy hide a new girlfriend from his ex?
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