"I love" and "I am loved" do not say the same thing, do they? The comparable passive voice for "I love [her]" would be "She is loved [by me]," not "I am loved [by her]."

No, they don't and I did not mean to imply that they do. I was just trying to illustrate that some constructions are periphrastic in that they have more than one word to convey a grammatical category. So, in English the passive voice is a periphrastic construction, "I am loved", but in Latin the passive is usually a single word, amor.

Ceci n'est pas un seing.