Petulia (1968)

When she sees him at a hospital fundraiser, San Francisco socialite Petulia Danner, who has been married to naval engineer David Danner for six months, seems determined to have an affair with soon-to-be-divorced physician Dr. Archie Bollen, who has been married to his wife Polo for close to 10 years; many thought they had the perfect marriage.

Archie doesn't even know Petulia's name; he only knows her as the woman who brought a young Mexican boy into the hospital with serious injuries. Petulia's pursuit of Archie is relentless; she even declares that they'll be married someday, even as he dates another woman, May. Archieeventually learns part of the reason why Petulia is so determined to cheat on David as her relationship to David and his wealthy father are brought to light. Archie also learns that getting involved with Petulia in any way has its consequences, both for Petulia and for him, most specifically in his relationship to Polo.