Two Males for Alexa(1971)

Overview:

A young girl marries the wealthy father of one of her male friends at college, but she soon finds herself falling for her new husband’s son. The husband finds out about the two, and comes up with what he believes to be a foolproof plan: he will frame his wife and his son for his own murder, then have the pair locked in a room in his remote, isolated estate–with his own dead body.