Dorayome

Dorayome (ドラよめ, lit. "Dragon Wife") is a one-shot manga by Ayumu Korai. It was serialized in Shonen Ace.

Plot
A dragon's hormones are not a trifling thing. Siegfried rolled a 20 for a chance to defeat the dragon (who was hunting for a mate) and accidentally hit the dragon with {Charm}, next thing you know they are married.