Economic Inequality
Polyamory and open/alternative relationships gaining acceptance is yet another example of this fragmentation. The expectation up until now is that you need a single "vertically integrated" partner that satisfies most of the partner roles in your life. More and more people are starting to realize that this is a compromise between time and satisfaction. With a single partner, you get a lot more of their time, but the likelihood that they will be satisfactory for all of the roles partners typically assume is less likely. There are two reasons for this that I can see.The first is that we are less and less likely to share extremely similar upbringing with others such that it is easy to find someone who is largely on the same page. When people were born, lived and died in a small town or city and the internet didn't exists, two people from the same place likely have a LOT in common. Now, everyone is shaped by the information they encounter and experiences they have, and at no time in history has there been as much variation between people than now.The second is that our sexual and romantic tastes are fragmenting and we demanding greater satisfaction in both breadth of tastes and depth of tastes. People want to have the same level of satisfaction from the romance and sex parts of their lives well beyond the honeymoon period, and when a single partner doesn't or can't meet the variety of needs you start looking elsewhere to satisfy them. The difference is now that there is acknowledgement of this greater fragmentation in sexual and romantic needs, giving people the position to be more transparent with dialogue surrounding it. There is also a greater acknowledgement of enthusiastic consent between all partners involved in an act as being important. This focus on consent means that you don't try to make someone participate in something they don't desire to participate, thus you need to find other partners that reciprocate for those sexual or romantic needs.