A French joke

At a seminar that I attended in Nice, the presenters for the graveyard shift after lunch decided to ask a very French question to encourage audience participation.
Accordingly, they asked everyone to stand up and then said that everyone who had sex last night was to sit down.
A few people, looking very smug, did so. The rest of the group looked around a little sheepishly.
"Well, who had sex one night in the last week?"
More of the audience sat down.
So who only got sex one night in the last three months?
More of the audience sat down.
"Who only got sex one night in the last six months?"
Now there was only one guy standing up and he was smiling.
Then they asked, "who gets sex only one night in the last nine months?"
The one guy stayed standing up and kept on smiling.
They then said "If you only get sex one night every 12 months, please keep standing up. "
The same guy continued to stand up, smiling.
The presenter said " You only get sex one night every year and yet you are still smiling. How could that be?"
Then the guy said, "It's tonight."

PS this was going to go on the end of the thread on Friday drivelling, but that thread got closed.