Well, the main difference is that child porn is illegal, but I'd argue you actually can compare the two, since yaoi can potentially involve underage characters, at which point is becomes both yaoi and child porn.

For all that people find homosexuality offensive, it's not illegal, unlike child pornography -- and that's illegal not because people find it offensive, but issues of child endangerment. (Now, you can argue that it's not REALLY "child endangerment" because the "children" are drawn and thus not actually real, but I kind of feel that's an even slipperier slope.)