Originally Posted by Libertine

Actually, it makes it a lot more understandable.

Sex is one of the main ways for people to relieve stress. When his wife had cancer, he undoubtedly was extremely worried, and probably spent most of the time he was with her supporting her. At the same time, his wife probably wasn't able to support and comfort him as much as she normally did, meaning he not only suffered from immense stress, but also lacked a good way to relieve it.

Caring for sick loved ones is hard. It's even harder if there's nobody to care for you. That's why it's very common for people to cheat if their partner is sick, even if they care about their partner more than anything.