I Married a Monster from Outer Space 1958

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.

Bill Farrell marries lovely Marge Bradley, but almost from the start he is acting strange. For one thing he doesn't drink any alcohol and even more peculiar, he almost acts like he doesn't have any emotions. A year into their marriage, Marge still feels that there is something strange about him. The thing that really disturbs her is the fact that after a year of marriage, she can't seem to get pregnant. One night she notices Bill leaving the house going for a walk and follows him. She then sees him meeting with the other aliens and soon realizes that it is not her fiancée. The alien "Bill" soon explains that the planet he comes from was destroyed and that he and the other males were able to escape. However, the females of their planet were unable to escape and were killed in the disaster. He then explains that the reason they are taking over the bodies of the Earth men so that they can mate with the women. A horrified Marge then tries to warn everyone of the alien threat.