Lets imagine it’s 150 years ago…
No internet, phones, TV or photographs, with minimal printed media, reduced levels of literacy and basic transport meant it would’ve been far harder for the average person to find other partners and more importantly, the lack of media would mean there was less reasons to be dissatisfied with your life as there was less points of reference with the outside world for comparison. There would’ve also been far less technology diverting peoples attention away from their sweetheart.
Obviously, infidelity and the like have been going on since way back, however, the tools now available to us make it much easier to pursue and find other partners outside of a primary relationship, with a magnified sense of personal entitlement and additional pressures of envy and discontent when shown the glossy sugar coated lives of celebrities.
So in summary, I basically blame the Beckhams.