[null and void]

Once upon a time I dated a woman who neglected to tell me she was married...

...to a Marine Corps drill instructor.

I always checked marital status after that!

[Lonesome in Massachussets]

Self preservation is one good reason. I also have an aversion to interfering with a marriage, including my own. I worked for a guy who was extremely charming and a real go-getter who was on his way to the top. He was married with two kids whom he adored. His wife started cheating on him with a musician. He made detailed plans to kill the guy in Excel ("The Spreadsheet Murder Mystery"), and followed through. He snuck into his rival's home and attacked him with a rubber mallet. Somehow, his intended victim managed to escape.

The case garnered some local notoriety and hilarity. He copped to attempted murder, and got a suspended sentence. The poor guy was very well liked, at work they kept him on the payroll on medical leave (which meant no pay check, but all his benefits including medical coverage were active) for almost a year, but eventually had to fire him. It literally ruined his life.

I know one thing. His kids will hate their mother for the rest of their lives. This shit is funny or harmless on inane movie or tv scripts, but terrible in real life.