I honestly think that is what the mangaka is setting up. We've kinda reached the point where Shingo and Keiichi have reached the most stable point in their relationship. And with all the hanky-panky she's been drawing, I think she's reaching her limit on what she can do to them. Besides, the love that Rio has for Shingo at this point is childish, but I also think Rio made a realization that he can't quite comprehend yet at his age, but understands enough: no matter how much he loves Shingo, he can't be his. I think as he gets older, his ideal of what love is going to be based on what his uncle and Shingo have.
Sorry for the rant, but I just had to get it out.
I love this manga so much. I wish to see Rio with his partner in the future :). Hope that there will be more stories like this one :D. ....They were so cuuute in that garden while holding their hands :).