I Married a Monster from Outer Space(1958)
OVERVIEW
Aliens from Outer Space are slowly switching places with real humans -- one of the first being a young man about to get married. Slowly, his new wife realizes something is wrong, and her suspicions are confirmed when her husband's odd behaviour begins to show up in other townspeople.
TAGLINE:
Shuddery things from beyond the stars, here to breed with human women!