External fertilization of eggs in dinosaurs probably did not occur because there are no known amniotes (reptiles, birds, mammals) that have external fertilization. External fertilization usually occurs in anamniotes (amphibians, fish) because there is water for the sperm of the male to swim in.
If you or anyone on this list has ever seen elephant mating, the penis of the elephant is like a mobile probicis that can be manuvered into position during the course of copulation. The reason for this is because even though elephants will copulate on top of one another, there is a great distance between the male and female genitalia.
In many reptiles, there is an internal hemipenis which again can be manuvered -- because the penis is bifurcated (has two halves), this affords the reptile with a hemipenis better chances of copulation with a female.
In some camels, the female lays on the ground sideways and the male mounts her sideways.
In some birds, sperm transfer from the male to the female merely requires a moment's contact.
Therefore, even though stegosaurs had plates and spikes which may have prevented mating in the standard mammalian way, soft tissue modifications to genitalia and/or different positions may have afforded copulation in these dinosaurs.
External fertilization, while an interesting idea, is probably outside the realm of probability for dinosaur copulation and fertilization because it is not found (so far as I know) in living amniotes.