After they saw your wife photoshoot try to relax at park bench Sunday (i picked 3h on 17:00).
Hmm, I did the couple at the end of my gameplay so it's hard for me to tell what else you need to do to trigger them, for sure you need all the photo session with your wife but you probably did it. The only other thing that come to my mind is the girl at the bridge in the park at night (bench 23:00), there is two or three event there with the couple. Also you should try call Marcus at the bench 23:00 because there is some talk about photo sessions. Did you did that two things?