People generally use slang terms when they talk about sex in public, because it's considered "dirty talk" and therefore inappropriate for some ears to hear.
This tradition of using innocent sounding slang terms to discuss sex began long ago, and whether you call it making whoopee, bumpin' uglies or simply gettin' it on, chances are you'd rather use slang to talk about sex than speaking frankly.
mental_floss put together a fun collection of 31 Adorable Slang Terms For Sexual Intercourse From The Last 600 Years which includes these entries that are impossible to read without smiling:
2. Play nug-a-nug (1505)
4. Play at couch quail (1521)
8. Put the devil into hell (1616)
21. Play at rumpscuttle and clapperdepouch (1684)
28. Have one’s corn ground (1800)
Because nothing says sexy like having one's corn ground!