Nobody knows how or why the curse began. Some say it's an ancient disease accidentally released during Antarctic ice melts, others believe it's true magic cast to curb population growth. In any case, it just appeared one day, in every man, woman, and child, and it afflicted them in the same way.
When a cursed being masturbated, orgasmed, or achieved any sexual pleasure of that kind, their bodies would change into whoever they were looking at when they achieved orgasm. When a guy masturbates, he turns into whoever he was fapping to. When two couples had sex, they'd essentially swap genders and body types. Any other ways you could think of, people were changing bodies left and right, without rhyme or reason.
People were warned all across the globe to refrain from sexual pleasure until a cure was discovered. But, of course, nobody listened. It was only expected for the curse to take a few days to a week to discover, but it's been 4 years now with no cure. People couldn't go that long without pleasure of some kind. Some people found a few loopholes. If you masturbate to nothing, and as long as you don't look at anybody, you'll be free from transformation. You could also masturbate to a picture of yourself, and you'd turn back into yourself. But those were the smarter and more resourceful people who figured this out. Most people ignored the warnings and the advice, and went along with their original lives, to crazy results.
Needless to say, life was pretty crazy for humans for the last 4 years, and would be for 5 more, until they finally discovered a cure.