That’s why the ‘of their species’ part about sexual maturity is important. As long as the Dragonborn is of consenting age for a human, it’s fine. Frowned upon? Maybe. Cultural taboo? Perchance. But as long as it’s consentual, you go and bone (or get boned) by whatever dragon you want
The Dragonborn speaks the dragon tongue, it’s fine
Perhaps, but Partysnacks is thousands of years old. That’s a huge age gap, even if the Dragonborn has a dragon soul.
That’s why the ‘of their species’ part about sexual maturity is important. As long as the Dragonborn is of consenting age for a human, it’s fine. Frowned upon? Maybe. Cultural taboo? Perchance. But as long as it’s consentual, you go and bone (or get boned) by whatever dragon you want