There's literally a hint system in the game?? I don't know if it was around when you played, but there's a system that tells you exactly what route to expect.
Also she isn't happily married. Her husband spends tons of time away from home. You clearly never been in a long distance relationship. For me, I'm constantly pent up thinking about my partner. And I've had temptations before I've had to ignore. She isn't in a good relationship for sure.