Most Helpful Girl
Our society went through what sociologists call the Sexual Revolution. Think back to the past and way things are now, sexuality is a lot more accepted and people are a lot more promiscuous than they used to be. In the past, if a man wanted to cheat, who would he have sex with? Most women waited till marriage to have sex, so his options were very limited and it was probably barely even a possibility to cheat back then. Nowadays, men and women are much more sexually active and there's a lot less stigma behind sex than there used to be. Sex is everywhere and it's quite easy for most to come by. I wouldn't say that it's accepted in today's society, but it's definitely more prevalent because sex is a lot more available, therefore creating much more temptations in one's life and making it harder to resist.
Most Helpful Guy
I think it is because of the secularisation (religion loosing power) also causing people to take lighter on the wedding wows. It's no longer a divine ceremony to many, it's simply a legal document that can be broken
So in effect there's less holding them back from breaking up.
This can be good, because while breakup rates go significantly up, it also makes it easier for people in toxic relationships to get out and just leave. Nobody will judge them anymore.
As for so many cheat; same reason contributes; no divine power watching over them. Furthermore society is more focused towards instantgratification, so people might be more prone to just cheat than to fix their relationship like you more often did before. Furthermore the presense of protection have reduced the chance of an accidental child to almost zero, so people are less likely to be caught that way.
Just some contributing factors.