Seriously, I mean, the very first example of a Half-Dragon in the 3rd edition Monster Manual was that of an Half-Red Dragon/Half-Troll... If you're so horny that you would even bang a troll, it just proves that dragons would also do it with ants, non-sentient rocks, plants, goblins, humans, elves, dwarves, horses, fish and beholders.
They're literally sex-obsessed geezers.
I sure hope that 4th edition dragons tons down the horny procreation-aspect that all dragons in 3rd edition suffer. If anything at all, other dragons should show contempt for dragons who do it with non-dragons, the same as we humans look down upon people who do it with goats or dogs.
Yuck, disgusting.