Where does the modern pressure to pair up and marry come from?

I’ve seen numerous questions here asking about how many people you can date, and whether it’s ok to date if you’re not serious about the person (whatever serious means—sometimes marriageable). To me, this seems like young people these days must feel such pressure to pair up, and move in, and marry. It also seems to be taking over the language of the dating life of recently singled middle-age folks, as well.

People seem to divide relationships up into serious ones, friends without benefits, friends with benefits, and fuck buddies. Just the fact that they have to categorize relationships like this, and at a very early stage of the relationship, speaks to me of some enormous pressure to know where it is going and what, exactly, it is.

Just for the sake of argument, let’s assume I’m correct that this pressure exists. Where does it come from? Do people put this on themselves, or is it coming from the outside (society, parents, or something else)? Like every relationship must have some purpose, other than fun, friendship or the like? Why do you think things are getting so much more serious these days? Could it be the so-called sexual revolution?