But not when it's said with a lisp.

Depends on the lisp, I suppose. If it's a real speech impediment it might not be voiced. But the th in a staged lisp (sociolectal cue for a male homsexual speaking), it's not even a phoneme that occurs in English. It's usually more of an interdental fricative than normal. More forward. Think Daffy Duck's kind of lisp.

Ceci n'est pas un seing.