Does the phrase "have your cake and eat it too" describe a man with a wife who sleeps with other women?

Yes. It does describe that.

No. That phrase describes something impossible.

By the way, is it the wife or the husband who sleeps with other women?

yes for both because the wife could also have another man...

People could use that expression under those circumstances. It can mean many other things as well.

No. The guy is not getting regular sex from his wife (see: marriage, wife), so the phrase using the word 'cake' is really describing perhaps an actual cake that the wife made? Eating is definitely what the guy does with the girlfriend, since wives don't generally like sex enough to have that be going on at home.

Only if his wife is aware of, and approves of, him sleeping with the other women...

Why would the wife sleeping with other women equate to the husband having your cake and eating it too? Does he get to watch? Participate? More details and pictures needed.