hmmm, after maryn's comments too, perhaps a bit more context:
The story is about the guy, who is not a horrible guy, but like most people, he is selfish and prone to rationalizations. Having married his high school sweetheart, he hits an eventual midlife crisis as they drift apart, and he comes to regret a lack of experiences outside the woman, who was his first love, the girl he lost his virginity to, etc.
Every relationship is a tradeoff, and all of us are, at times, inclined to be selfish about what we gave up--he doesn't recognize it as a midlife crisis, and maybe it isn't, but he becomes obsessed with the notion he needs to know what he missed and was cheated out of by skipping the usual high school and college fuckaree, so he decides to find a girl for a fling, convincing himself it would be a one-time thing and that he just "needs to know". And in fairly short order, he begins to cultivate a relationship he intends to turn into a one-nighter.
While he's doing this, the wife learns she has cancer, and when she tells him, he takes the bits he wants to hear, like high cure rate, and reluctantly decides to "stick it out" until she's cured and he can get on with his life...sort of a "why does all the bad stuff happen to me?" thing (again, he's more than a little self-absorbed).
I need it to be a cancer with a high cure rate, so he isn't resigned to holding her hand till she dies, just to the notion he's going to "take care of her for a few months until this mess is over", but it also needs to be serious enough he needs to stay; having a skin biopsy and negative results in a week, for example, wouldn't be enough to make him stick around. And, in the end, she needs to actually get worse, but not over a five or ten year timeframe....
hope that helps flesh out the rationale for my parameters....