Scott was absolutely taken advantage of. But, as you say, he isn't blameless. Scott should've known better than to turn to Emma and play along with her seduction. Yet, he did it anyway.
No, Scott isn't exclusively to blame. IMO, Emma is the most culpable, followed by Scott and then Jean. (Jean's role is the weakest part of the whole affair storyline; Morrison never showed on-panel just WHY Jean wouldn't reestablish their psychic rapport and wouldn't reach out to Scott, leaving it to us the readers to infer an explanation.)
Once again, though, all of this is water under the bridge. As a plot point, the affair is a dead letter. Scott and Jean have already made up and reaffirmed their love for each other. What's left is for them to rebuild their relationship. That's the story I want to see. Some discussion of the affair should be just the start of that. A circular firing squad blame game would just come off as cheap and petty and, IMO, wouldn't be interesting anyway.