Typically, when I eat out, it's with my husband, and he knows that if he wasn't told we're sharing, he gets a bite, unless I decide I don't want the whole thing. I have gotten vaguely irritated when I order a dessert, and it's put halfway between myself and my husband, with two spoons. Same thing has happened in reverse, but not as often (usually if only one of us is getting a dessert, it's me).
I haven't seen a server say it was for sharing, but I have seen them give each of us our own silverware, and put the dessert in the middle, and indicate it's for the both of us ("Here you two go" or something on those lines.) I don't dock tip on it, but then, it usually isn't as blatant as Dotty's situation because it's either myself and my husband, or myself, my husband, and my brother in law, or my brother. People I don't have any problem defending my dessert with, basically, rather than an obvious gathering of non-related individuals.
I doubt that makes it any better, but it does mean it doesn't register as much, due to the fact all the individuals at dinner know better than to go for my dessert without at least asking first.