He fell in and out of love with Ayn Rand. He may not have fully fallen in love with her but they did have intercourse while both were married to different people. Physical and intellectual love between a man and a woman is a complicated business. Nathaniel and Ayn were no different than plenty of other people who have been in love with two people at the same time. It’s far less than ideal but it happened and it was certainly wrong by many different ways to calculate the wrongness.

Nathaniel’s big mistake was neglecting the middle of relationships. A person in love with another can be in love with different people at the same time. There are rules about such relationships but many of those rules are routinely broken. Nathaniel broke a few while he was married to Barbara Branden and having a sexual relationship with Ayn Rand. Ayn had to stop the relationship to work on her monumental novel Atlas Shrugged. Nathaniel found and became sexually involved with another woman, Patrecia Scott (née Gullison).  At that point Rand wanted to resume the affair but Nathaniel wasn’t able to do it.  Nathaniel moved to California with Patrecia who was killed in an accident.

Branden subsequently married businesswoman (and later psychotherapist) Estelle Devers in December 1978. The two later divorced, though they remained friends. Branden subsequently married Leigh Horton.

Branden also retained a relationship—sometimes friendly, sometimes acrimonious—with his first wife, Barbara.