User blog comment:Oppaichan/Rito and Lala still engaged?/@comment-12814990-20140105153344/@comment-7948498-20140105204953

Yeah, but at that point, Rito just barely got his feelings sorted out, let alone his intent to marry any of them. And even thus far in To Love Ru Darkness, Rito still didn't agree to marry any of them. He's too damn slow to get past the early early stages of love.

I think Mikan was aware of the engagement, but didn't really take it seriously. She probably figured to let them play around to make things more interesting.