Well, when you think about it logically, it really isn't odd at all.
Animated characters are designed to be visually appealing. If the character design of a cartoon is ugly, no one will watch it. Thus, it's entirely normal to find a character design appealing and, well, attractive. It looks good. It's attractive. In many cases, it's recognizably female, as well; while I think it's about impossible to literally find ink-on-paper attractive, some people can easily "conceptualize" what a character looks like. Jessica Rabbit, for example, isn't particularly human-looking. But people can "smooth out the edges" of the character into a more realistic concept, even subconsciously.
Hard to explain what I mean there.
On the other hand, characters are also designed to be endearing, in and out of animation. If you don't have an emotional investment in a character, it's a failure of a character, more or less. These characters are intended to be as human as possible.
Human beings are generally attracted to other human beings, be they fictional or not. What's the point of romance novels if the tall, rugged, handsome cowboy ISN'T supposed to be found attractive?
So combine A) aesthetic appeal, B) endearing characterization, and C) basic human instinct to find other human psyches attractive... and it's only common sense to find animated character attractive.
I think the main difference is in how easily an individual can smooth out the edges. Some people can't see a cartoon character as anything but ink-and-paper. Some people can quite easily see a cartoon character as an abstract representation of a "real" individual. And some people are so intuitive in this manner that they can find anthropomorphic fish attractive.
That's just my thoughts, though.