How did Stephen's behavior contribute to the decline of his marriage in the novel, The Wallcreeper?

In the course of their relationship Stephen had an affair with Birke, slept with Tiffany’s sister, slept with Omar’s wife and abused drugs on a regular basis. Tiffany began to suspect Stephen was going on his birding trips alone just so he could use drugs as much as he wanted. It was noted he would always suffer withdrawal when he came home. Although Tiffany didn’t learn Stephen had a bad heart until one of the last trips they took together, she realized it had affected him all of their lives together.