"Did T. rex have a penis? Did he even, as lizards do, have two?
I ask the question not out of prurience, but because it's a matter of scientific interest. There are a couple of reasons why. First, the penis is another important indicator of the mating system. In species where females usually mate with a single male during a breeding episode, penises tend to be small and uninteresting. In those where females mate with several males (whether by choice or by force), penises are typically larger, and come with fancy decorations such as grooves, nobbles, and spikes. Second, the question of the dinosaur penis provides an exercise in evolutionary inference.
The reason we don't know whether T. rex had one is that the organ is generally too soft to leave a fossil trace. (There's an exception to this: some mammals have a bone in their penis, the os penis or baculum. This can fossilize. Humans are unusual among primates in not having one; in case you're wondering, it's not clear whether the bone plays a role in maintaining erections.)
Moreover, whether a male has a penis at all varies from one group to the next. Male salamanders, for instance, don't: they deposit sperm on the ground and the female collects it. Among birds, penises are rare: ostriches, emus, ducks, geese and swans are among the few. The rest just have a cloaca — an all-purpose opening also used for urination, defecation and, in the female, laying eggs. To copulate, two birds bring their cloacae together in what's called a cloacal kiss.
So what can we say about dinosaurs? My guess is that the males had members — but it's an educated guess
..." NYT