I know we collectively agree that Hiccup isn’t romantically inclined, and his getting married and having kids didn’t make sense in the epilogue, but consider: Hiccup getting married for political reasons.

It’s a marriage of alliance, which is recognized both by him and his partner, and they enter it without expectations of romantic involvement. Since they’re now married, they live in the same castle, spend time together, and Hiccup finds he really likes his spouse. They’re funny, get along with his friends, and has the same interests and values. They both probably speak multiple languages. She understands why Hiccup is so dedicated to making the Wilderwest better, and holds similar views. She’s a good politician (her job after all, was to be an ambassador). Hiccup likes spending time with them, and the feeling is mutual. They’re not in love, they have their own lives, but they’re dedicated to each other and eventually decide to raise children. They teach their kids how to train hawks and hunt with dragons, riding, history, the Languages, and all the necessary skills of their world. They’re not in love and they’re happy together.