My Husband is a Doomsday Weapon.

A young scientist working to restore a civilization destroyed centuries ago by cataclysmic war unearths a devastatingly powerful doomsday weapon—which saves her from a marauding robot, then promptly transforms into human guise, identifying her as its new master. Thanks to the input from a romance novel, this handsome anthropomorphic weapon dedicates itself body and soul to our heroine, who in turn realizes that this may be the answer to her total inability to interact with human men...

(Source: Kodansha USA)