Sex and relationships don't seem as important to people today. Life and society has changed drastically over many years. Men are suspicious of women in many cases. Women have become more successful and independent having more individual success and of course higher earnings. Men aren't needed as they were many years ago. Plus when relationships fail after marriage, most times the laws favor the women and it can ruin men financially for many years. Women have become more vocal in their likes, dislikes and demands, thus men began to view sexual relations as being performance based rather than a fun and loving activity.
Many years ago there just wasn't as many things to occupy our personal time. Now we have advanced communications devices, TV, PC's, cell phones etc, that connect us to the world, isolation caused by such things resulting in lose of interpersonal skills, which basically means we don't know how to interact with each other. Many years ago none of these devices existed so there simply wasn't as many things taking up our time so the next best thing was there was more time for love making. Also, scientists have noticed in recent years testosterone levels in men and boys have been dropping for many years. Who knows why that is, but the end result is the lower the testosterone, the less likely males will want or need sex.
I'm also guessing that church attendance has fallen and there seems to be a certain level of moral decay, thus relationships lack the respect we've once had for each other, the value of the relationship and the individual. It's too easy just to give up and move on when things don't go as expected, rather than communicate and find answers to what's wrong and fix it. It's all about total respect for your significant or even insignificant other.