Not sure that marriage has anything to do with understanding the opposite sex in itself. I assume you mean having a sexual relationship? In fact, most people seem to learn nothing from a sexual relationship, even one spanning decades. A man climbs on top, does his thing & rolls off. Women are indifferent. Men tend to think that sex is the be all and end all of marriage. Women focus more on the companionable aspects that men tend to neglect. So each side feels that the other does not understand their needs. Over time the differences between men and women can become more apparent if one is analytical about it. But few people are - it would seem.