They don't even bother with the pushups that tend to occupy their more solitary hours. The female is more alert, but they're both just kind of sitting there. It takes a good five to 10 minutes for sperm transfer to take place. The penis is a dartlike structure, and they actually shoot each other. They exchange sperm in both directions and both produce offspring.
Barnacles mail it in, too.
Only they don't care where it goes. The larvae tend to settle on rocks by their heads, then secrete a shell and are permanently cemented to those rocks. It turns out that every barnacle has a penis 14 barnacle diameters long. So within that radius, in any direction, most barnacles have a whole lot of possibilities on Saturday night. You'll have a robust bunch of kids.