Not quite Gilligan's Island

A charter cruise on the South Pacific goes horribly wrong, the ship sinks, and there are only 3 survivors; Jim, Tom and little ol' Susie. The old geezers manage to swim to a small island and they live there for a couple of years doing what's natural for men and women to do. After several years of casual sex, almost daily to break the boredom, Susie felt absolutely horrible about what she was doing. She felt having sex with both Jim and Tom was so immoral and bad that she killed herself. It was tragic, but old Jim and Tom managed to get through it. After a while, Jim and Tom's resistance to nature's urgings waned, and the inevitable happened. Well, a couple more years went by and Jim and Tom began to feel absolutely horrible about what they were doing... So, they buried Susie.