What's the difference between 남자에는 & 남자에게는?

In his book 국어의 풍경들 (Views of the Korean Language), Ko Jong-seok (고종석) introduces the following Korean riddle:

남자에게는 있고 여자에게는 없는 것, 그러나 엄마에게는 있고 아빠에게는 없는 것?

Before clicking on the answer HERE, try to figure out the riddle on your own.

Did you figure it out? If not, why not? Would it have made any difference if 에는 had been used in the riddle instead of 에게는?

In the article in his book, Mr. Ko explains that the riddle is not really fair since 에게는 was used instead 에는. The grammar rule is that 에게 is used with animals or people, and 에 is used with plants, places, things, ideas or time, so since the riddle was treating 남자, 여자, 엄마, and 아빠 as words (things) rather than as people, 에는 should have been used with them instead of 에게는. The reason 에는 was not used is that it would have been too big of a clue for observant Koreans.

Here is one more riddle:

하늘과 땅 사이에는 뭐가 있을까?

The answer is HERE. I bet this one is easier.