So I'm 23m and have slept with about 6-7 girls with partners and after speaking with them all throughout the years I've come to find that most of them all said the same thing basically that they love their partner to bits and everything about them but they are just terrible In bed and I think people associate love and physical pleasure as the same thing when in reality they are completely different things and needs for 95% of people. So I do believe that people still do love there partners but like anything it can get abit stale and boring for people sometimes doing the rinse and repeat in the bedroom. Me and my ex were extremely toxic (now we broke up for different reasons) but I remember we started having 3sums/4sums with other people a long time ago a couple times a month and we were the best we had ever been and all we changed was adding people to the bed room. Basically what I'm saying is I don't believe people who cheat don't love their other significant partner and just need to spice things up to keep the relationship healthy sometimes (this is not for everyone but from the people I've been keeping an eye on over the years I definitely notice a pattern over the past few years)