I was 8 when my dad married my stepmother, and no one had ever talked to me about sex (nor would they). When they got in the car to leave on their honeymoon, I didn't understand why we weren't going with them. One of my brothers told me they would be "doing their taxes". Dad and stepmother heard about it and liked it, so for the longest time any time they'd disappear into their bedroom for a long time they'd say they were doing their taxes!

What do you call it? I usually just say "doin' it" or "having sex" but I found this fun list of 165 sex euphemisms.

NSFW, obviously:

http://www.amog.com/lifestyle/sex-euphemisms/