I forgot the title of the manga where the hero and demon king/queen time traveled and wound up near a tree. The male hero has amnesia, but the demon lord states they are married.
This is Kiraware Yuusha wo Enjita Ore wa, Nazeka Last Boss ni Sukarete Issho ni Seikatsu shitemasu!, aka I played the role of a hated hero, but for some reason I'm loved by the Last Boss and living with her!
Together with his companions, the hero Ares was facing the final battle with the demon Demiurge. However, because of his habitual misbehavior, he is alienated from his three companions just before the battle, and he is left to challenge the final boss fight alone.
Against Demiurge's powerful barrier which repels all attacks, Ares takes a final step in exchange for his life, but...
Love will save the world! What shocking vision of the future awaits the hero Ares?
Ares fights the demon lord alone after convincing his companions that he's scum, and tries to sacrifice himself to defeat it. Then it flash forwards to him waking up in the middle of a forest, naked, with a woman who claims that she's the demon lord he just fought, and insists they're married.
It's not quite amnesia, but he doesn't remember anything that wasn't shown. He expected to die, and initially thought the forest was the afterlife. I'm also pretty sure there was no time travel, just an ordinary time skip.