I need to preface by saying I'm not condoning that baby line, but I THINK I get what they're talking about. When you look at a baby's eyes or smile it can potentially remind you of a lover's eyes or smile. The baby itself is cute but when you consider a plain girl is 50% erotic and a baby is considered 3%, by suspicion is the baby ITSELF is not erotic (and if you do, please hold your head underwater for an hour) but they are erotic by proxy.
Just my theory. But again if you DO find a baby itself erotic, do society a favor and slit your throat.