I don't know why, but the general reaction (it seems about 90%) to the photo is that people see her as looking more coquettish than innocent. It's most likely enhanced by the juxtaposition of him kissing his wife (and we know all know how slobbery that is) at the same time. Then Tipper seems like she might be headed out the door after getting him all warmed up.
Prior to this photo, AlGore has always been seen as harmlessly asexual, and not a perv or anything. That's why he did that Rolling Stone cover and staged that make-out session on live TV to look more manly.