So there is a ritual where royalty gives their official consort (NOT a concubine) something called a "King's Flower" which is a flower tattoo that appears on a consort's chest. The King's Flower is the only way a royal can permanently contain their dangerous magical power. If they fail to find a consort to give the King's Flower to, they will literally explode from the magical energy when they reach adulthood. The ritual requires the consort to be a virgin (which is why Lidi planned losing her virginity so she could avoid marrying into royalty). A royal can only do the King's Flower ritual once, so it's very important to find the right person to receive it because if his partner's not a virgin, the ritual would fail, he would be unable to do it again, and he would die.
The prince at this point is already halfway in love with "Diana" (because he imprinted on Lidi as his mate at first sight), and after discovering "Diana" is a virgin during their foreplay, he decides then and there to make her his consort and give her the "King's Flower". So he takes her virginity, giving her the "King's Flower", without Lidi even realizing what he's done.
After he falls asleep, she leaves him in the middle of the night to sneak back home, patting herself on the back and believing that since she lost her virginity she no longer has to be engaged to royalty.
But surprise! Apollo is Prince Freidrich (gasp) with dark hair he dyed with magic. When he wakes up and finds Diana no longer next to him, he goes into full search mode and demands his associates to bring a portrait of every young daughter of nobility, trying to find a girl with violet eyes. His knight commander Glenn asks what about his fiancee, the prime minister's daughter, and Freed says he doesn't care about his fiancee and all he wants is the mystery girl that he gave the King's Flower to (ha!).
After going through all the portraits and coming up with nothing, he notices Glenn taking away a portrait he hasn't seen yet. Glenn was taking it away because it was the portrait of his fiancee that Freed announced that he wasn't going to marry. Freed looks at Lidi's portrait, realizes it's her, and runs off to the Prime Minister's residence for a sudden "fiancee meeting". He meets Lidi, and Lidi has no idea that Freed is Apollo.
After they go to her room away from her parents, Lidi tells Freed that she can't be married to him because she's not a virgin. Freed then reveals that he was Apollo and her virginity was no longer an issue because she lost it to him. He says he absolutely wants her to be his princess consort no matter what. He reveals that he gave her the "King's Flower" during sex, explaining the ritual requires penetration, the partner being a virgin, and (ahem) coming inside her. She's shocked and tells him she doesn't want to marry him because she absolutely doesn't want to be in a polygamous marriage. Freed then tells her that was easy because he doesn't want any concubines, just her. With no more arguments, Lidi finds herself swept away in the prince's flurry of lovey doveyness.
There's a lot more technical details like contraception and the like but going into more details will make this even longer (Please rejoice that Lidi is a smart girl who got her own protection before she had sex...be safe, ladies!)
The story goes on from there with a lot of great characters that show up (and a lot of sexy times XD)