cleave to (one)
To have sex with one (typically a spouse) only, and no one else. I would never cheat on my husband—after all, I vowed to cleave to him until my dying day.
cleave to someone
to be sexually faithful, usually to one's husband. (Biblical. As in the traditional marriage ceremony, "And cleave only unto him.") She promised to cleave only to him for the rest of her life.