Update: As of April 2016, the last two hearts are greyed-out for marriage candidates until you give them the bouquet. This question is obsolete.
I'm super about completion, and I've been grinding up the relationships with pretty much everybody. I married Haley quite some time ago (we just had our second kid) and I have her at 12, Penny was at 10 (she is currently 9) and Elliot is flopping between 10 and 9. Everyone else who isn't marked single is at 10 (except for my son), while all other single villagers are at 8. I have seen the 10 heart events for both Elliot (told him he was making me uncomfortable, because married to a girl) and Penny (told her I didn't like her that way, because I'm married, you homewrecker).
Considering the sheer amount of favorite gifts I've thrown at these people (and the implacable decay I've encountered with Elliot), I'm beginning to suspect that there was an update at some point that made sure you can't reach 10 hearts with a single villager who isn't your spouse/bouquet recipient. Can anyone confirm from an official source?