To be honest it probably has to do with the sexual market. We subconsciously want our genes to be passed on in the most successful manner. Our sons can pass their genes on to many women at once (if they wanted to), thus due to their high sexual supply they are in low sexual demand. We know that our genetic lineage, by statistics, will be secure because some of those women are bound to have good genes that ours will combine with to make the perfect progeny. Women can only reproduce with one partner at a time, though, so we are sexually protective over them because we want to ensure the best reproductive success subconsciously.

Its the same as how certain physical features are considered attractive not because they are objectively attractive but because they have become instinctual markers of genetic quality. We seek out attractive people because we subconsciously want our kids to be attractive and want them to continue the tradition of having attractive children of their own, thus ensuring their reproductive success.

Creepy, huh?