Spend "quality time" at Hiccup's house.

As soon as the door to the front door was closed, Jack immediately pinned Hiccup to it and stuck his tongue in his mouth. The desperately rubbed against each other through their clothes as they clumsily wandered up the stairs attached to the hip.

Hiccup's father Chief Stoick was at the Great Hall and Toothless was curled up on the roof, so they should have the privacy of the home all to themselves. They were both dressed in their official flight suits -- both made of leather, Hiccup's done in red and green while Jack's in white and blue -- and helped each other undo the straps that held them together. By the time they crashed onto Hiccup's mattress (which was more like a plank of wood and a yak-fur pelt for sheets), they were in nothing but their skivvies, Hiccup's prosthetic leg strapped to his upper thigh.

Jack rubbed his painful erection against Hiccup's with nothing but two layers of fabric keeping them apart.

"Mhmm, I love you," said Jack in between kisses.

As Hiccup fought nervously to say it back, it was interrupted when he felt Jack's hands -- which were always inexplicably cold -- reach under his skivvies and grab what was under it while his tongue crawled over the viking prince's throat.

Jack carefully undid the straps keeping the prosthetic attached before pulling off the other boy's underwear, revealing his throbbing member. It was longer than people believed the Chief's son would have, the foreskin desperately clinging to it for dear life before Jack peeled it back. Without hesitating, he took the head into his mouth. Hiccup was overwhelmed by the sensation and couldn't help but cry out, hoping there were no patrols out in the quiet village who heard him.

Hiccup had read about the act of sex -- orally and otherwise -- from an absolute treasure-trove of old Roman poems and sagas imported from Constantinople by Trader Johann. They described how intoxicating it could be, but Hiccup never imagined how much he would like it until he and Jack's first time.

It was also in these sagas that he learned that he was not alone in his attraction to men. Safe to say that when Jack found his hidden stash of sagas, Hiccup was embarrassed. Granted, he and Jack were already a couple by then and Jack couldn't read Latin, but the pictures were graphic enough for Jack to understand what they were. By the time they advanced their relationship into the physical, Jack had Hiccup read them to him just so they could try all of them one by one.