In the first scene in the play, Emma says that Robert has betrayed her with multiple other women for years.
Emma, Robert's wife, meets with Jerry, Robert's best friend, in 1977. She tells him that she and Robert are divorcing and that he's been seeing other women for years. Jerry points out that he and Emma were in an affair behind Robert's back for years. Still, though, Emma sees Robert's confession as a massive betrayal.
Jerry and Emma saw each other for years without Robert knowing. It's been two years since they ended their affair.
However, the audience only has Emma's word that Robert confessed to having affairs. Emma is an unreliable character who has lied to Jerry before. For example, she told him that she only confessed their affair to Robert the night before when really she told him the truth years ago. He knew and baited Jerry about it; he later told Jerry and reassured him that it was okay. Jerry is shocked when he finds out about Robert's affairs and tells Emma that despite the time he spent with Robert, he never suspected.
Ultimately, though, Robert did sleep with other women. It's probably one of the reasons that he was accepting of his wife's affair with Jerry. He found his own partners on the side. This is something that Emma is ultimately unable to cope with despite her numerous affairs.