@maverih345456, Yeah, I'd have no questions about the story if it wasn't for that second to last page. Why did she become an adult version of her female self for one page(which I also would like to have a seen a few more pages of)
There was not age change for the girl in the black hair, just the way the artist drew her (hair tied and no tied)
Otherwise story is the girl gave the guy some Valentine chocolate with a Love Potion recipe from her older brother, but in fact was a gender changing recipe instead. The girl, worried that the ex-guy would lose his virginity by strangers, decides to eat the chocolate as well and take his virginity instead.
They turn back afterwards because the drug only works for 2 hours.