Always wondered about this.
"Ever think about dinosaur sex? Paleontologists do. And they've come up with some surprisingly specific ideas about how the prehistoric beasts were able to mate despite their enormous size and weight--and despite the horns and other bony appendages that might have proven bothersome when the creatures got hot and bothered.
The males and females of modern-day birds and reptiles have a single body opening for urination, defecation, and reproduction--something called a cloaca (Latin for sewer). Paleontologists believe that dinosaurs had the same basic equipment, and that they coupled by pressing their cloacas together.
No penis is needed to perform a "cloacal kiss." But some birds have penises and crocodiles sport penis-like "intromittent organs," and male dinosaurs might have had something similar. As you might imagine, a dinosaur penis might have been pretty big--perhaps up to 12 feet in length for T. Rexes.
But how did Mr. Dino ever get his cloaca near Ms. Dino's? By mounting her from behind. At least that's the view of many paleontologists, including one Beverly Halstead, an Englishman who became known for his candid talk about dinosaur mating before his death in 1991. For an article that appeared in the now-defunct science magazine "Omni" magazine in 1988, Dr. Halstead said:
All dinosaurs used the same basic position to mate. Mounting from the rear, he put his forelimbs on her shoulders, lifting one hind limb across her back and twisting his tail under hers to align the cloaca.
Some of Halstead's present-day counterparts concur that that's the way dinos did it.
"I don't think there's much doubt about that," Dr. Gregory M. Erickson, an evolutionary biologist at Florida State University, told The Huffington Post in a telephone interview. But, he acknowledged, "It must have been a hell of a thing to see."
it's a big animal thang