‘You and me baby we ain’t nothin’ but mammals. So let’s do it like they do on the Discovery Channel".
Bloodhound Gang.
He was enjoying that a lot more than she was.
That was both cute and disgusting. 
At one point I wanted to tell him to breathe! (He did, he’s fine.)
Yeah, not long ago I was walking the dog and came across two lizards “doing it,” and paused, staring. I tried not to interrupt but they sensed us, broke apart, then fled. I felt like a guilty peeping tom.
I’ve seen several of the scenarios already mentioned; as a 10 year old I was having dinner with my family at a fancy restaurant when we observed the mallard gang rape described earlier. My dad tried to be blasé while my mom just wanted to sink into her chair.
As a preschool teacher I’ve taken many field trips to the zoo during spring time, and the mating scenes are inevitable: peacocks displaying, turtles mounting etc.
The topper was a baboon twosome, where the female was “presenting” with an interested male nearby. Between the interesting body parts and charged atmosphere, there were a few embarrassed parents and teachers fielding pointed kiddie questions.
Gerald Durell’s My Family and Other Animals has a laugh out loud description of mating tortoises.
Bourbon Street Bar and Grill in Bangkok, a good, long-running place and where we always go for our Thanksgiving meal, including this year, has a series of photos lining the men’s room, each one showing different animals doing it, everything from snakes to elephants. The wife says there’s nothing like that in the ladies’ room.
We saw some gators going at it a few months ago at the zoo. Sounds, splashing… kind of scary actually. My 2.5 year old was a little frightened and asked what they were doing. I said they were playing 
I’m THINKING it was in John Sedgwick’s The Peaceable Kingdom, an annal of the Philadelphia Zoo, that there was a chapter about a sex-crazed tortoise who used to mount not just lady tortoises but dome-shaped rocks, overturned buckets and similar almost-tortoise-shaped objects and just thrust and moan for hours.