Door#2: Rocky Relationships
Marriage can have its own stressors without having to add the power of the Presidency into the mix. Mary Todd and Abraham Lincoln reportedly had quite a stormy marriage. The issues in their marriage were numerous, starting with their very different upbringings, as Mary was a child of means and Abe famously grew up in poverty. They had different ways of dealing with arguments; Mary displayed a violent temper. She would throw things, and Abe would silently just walk away.
In the role of President, Abraham would travel six months out of the year, which Mary perceived as abandonment. Finances were also an issue: Mary often spent beyond their means and used shopping as a way to console herself. Both of them are reported to have had issues with depression. They remained married, however, and Mary was so bereaved when Abraham was assassinated that she didn't even attend the funeral.
Lyndon and Lady Bird Johnson were married for 38 years but that didn't translate to an issue-free marriage. Lyndon was known to be difficult and would embarrass Lady Bird in public by commenting on her attire and flaunting numerous affairs with other women.
And, affairs are a malady that has stricken many a Presidential union, which we find behind Door #3.
It's unfortunately much easier to find examples of Presidential power and influence making it hard for a marriage to remain stable and for the Commander in Chief to stay faithful. As power is the ultimate aphrodisiac, there are many women who remain amenable to a dalliance with the wealthy and powerful, regardless of marital status.
Reports of affairs go all the way back to Thomas Jefferson, whose affair with Sally Hemings, a slave he met in Paris, led to the birth of five children and now-notorious birthright demands from her progeny. Other Presidents followed suit, from affairs with White House staff (Franklin D. Roosevelt, Dwight D. Eisenhower, Bill Clinton) to multiple affairs and dalliances a la John F. Kennedy. Warren Harding's infidelity may have been his undoing. While the official record is that he died of a heart attack, it's rumored that his wife poisoned him.