The good part is that I've been messing with trouble-shooting tools and finally found the words to the motivation equation (or how to find what you're missing).
When characters come into contact with a story event, they react to it according to the events and socio-economic factors shaping their lives, and that reaction in turn becomes the backstory (as the character moves through the story) on which to base future reactions. In other words, you can always tell where you’re going by where you’ve been. Which is a good thing when you’re looking for motivation, because the equation runs both ways, i.e., if you know what you want, you can find what needs to be in place to order to get it. If you know backstory + story events, you know what's going to happen. People aren't like random number generators. It's highly unlikely a strait-laced woman who doesn't believe in extra-marital sex will have a sudden sexual encounter with someone other than the man she eventually marries, if factors in her backstory haven't already set up the possibility. It's more likely she'll continue on her path, and on coming into contact with a story event (temptation, maybe?) act in a way consistent with her character.
However, if you want her to have a random sexual encounter--then the seeds of that act need to be in backstory. Perhaps a repressive upbringing, depressed, sickly mother, concealing clothing, home-schooling and a yearning to be like the public school kids.
A woman with repressed childhood, who was forced to wear concealing clothes, home-schooled and kept away from other children growing up but who yearned to be like public school kids + an opportunity to have blazing hot sex with a bad boy hero = a smoking hot sex scene
The same woman who did "not" yearn to be like public school kids and was okay with her life + an opportunity to have blazing hot sex with a bad boy hero = a total brush off
when you throw the same story event at different people, they react differently, which makes the story flow differently.
backstory + story events = desired results
desired results = backstory + story events