Yup... it's completely avoidable and totally your choice. Which means that if you get the wife to cheat on the husband, that's ALL your fault! Blame your own choices for that!
My spoiler was based on the choices in a few different saves I have scattered. From my playthroughs, the timid/bold saves do make a difference with how far she goes at the end..
Key words... either have her accept the trainer's advances OR reject those advances. But again, from my playthroughs, the bold/timid choices affected how far she went.