Whats "crossing the line" with a married woman?

Honestly, there are more ways to cheat other than just physical cheating. If some sort of emotional bond forms between two people, they can cheat emotionally. This is sometimes worse than physical cheating because then there is actual interest and attachment to the other person, whereas physically cheating is sometimes only just for the sex. Too much interest in any other person in a relationship is going to cause trouble. So, it is possible to "talk to much" and cheat.