Lately this is the fantasy that's been playing in my head. It's sort of like "the great shift", except instead of people swapping bodies with the nearest person, every person on earth flips genitals.. Meaning boys don't become complete girls, but they get boobs and a vagina. Girl's butts and hips don't get any smaller, but they grow a cock and loose their boobs.
In other words, the body shapes don't change, hair doesn't change length, just the genitals change. So imagine the wave hits, and you're with your girlfriend about to go to sleep.
Your bodies change, and your girlfriend looks at you with a smile because she has her first erection and the feeling is so strange yet so good at the same time. She looks deep into thought
Then you look down at your own body, shocked by the breasts staring back at you. You feel down at your crotch to realize your cock is missing
You stick a couple fingers in your new vagina, not being able to comprehend this foreign feeling
Just then your girlfriend slinks over to you and begins rubbing her hard cock on your pussy, and it's turning you on in ways you've never felt before