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to pledge; to vow; to promise; to swear
to have sexual intercourse (esp. between husband and wife); to share a bed
二人は永遠の愛を契ることにした。
The two decided to pledge eternal love.
友情を契って固い握手を交わしました。
We pledged friendship and shook hands.
神の前で永遠の愛を契る儀式だった。
It was a ceremony to vow love before God.