They were so happy with their spouses...
They both seem like shallow people that have extraordinary actors giving them more credit with the audience that they deserve.
Deniro has a beautiful, loving wife and 2 little kids but he dives into an emotional affair with this stranger and throws away a perfectly good family for nothing.
Streeps's husband is devoted and kindhearted but she drops him without any regard for his feelings. When her father dies she is more concerned with her would be lover than him. She runs away like an immature teenager.
If the luminous Streep and charismatic Deniro were not in the lead roles this movie wouldn't have a chance.
The script is underwritten and the main characters seem like disloyal jerks.
Even the 2 greatest actors of their generation can't make me root for them overall but it is an interesting curio from the 80's. They both infuse their roles with charm even if the script lets them down.
Incidentally, the movie only really comes to life when Jane Kasmareck lets him have it in her brief scene at the end.