I think most of us expected this, and possibly even that she thought of being a second with Saki. Shino knew the pitfalls of a polyamory relationship including the legal ramifcations wayyyyy too well, and people generally do not know that unless they have looked into it and/or discussed it with others in the community who are familiar with issues. The hospital one is pretty major for instance, same with taxes for people in mono relationships compared to tax benefits for people in multiple relationships.