Is friday night sex really a double mitzvah?

Commenter Batsheva sent me this question and I thought it was both funny and thought provoking enough to share here.

So here is something I’ve always wondered, but never had the guts to actually ask anyone.  Maybe it’s common knowledge.  I didn’t go to yeshiva, so there’s a lot I don’t know.  WHY is it a mitzvah (double mitzvah, I’ve heard said) to have sex on Friday night?

My understanding is that HaShem STOPPED creating life on the 7th day.  Somehow it doesn’t make sense to me that we’re not allowed to write a word, draw a picture, or light a match on the 7th day, but we’re allowed to perform the act that ultimately brings a new human being into the world. And yes, I know that the former things are considered work, while the latter is decidedly pleasurable, but let’s be honest here. You ever know anyone to work up a sweat writing a word on a piece of paper?

It seems to me that the real issues have always been creation and destruction that are forbidden on Shabbat (I think I even learned once that the 39 av melachot were all activities involved in the creation of the Beit Hamikdash).  So if creation is the issue, isn’t sex the ultimate act of creation?

Yes, I know that procreation is a mitzvah and it’s pleasurable, but giving tzedakah is a mitzvah and going to the movies is pleasurable, but I’m not allowed to do either of those on Shabbat (although we give tzedakah beforehand).  So what’s the deal here?  Why the big exception?  Is there really some logical reason I’m missing, or was this just Hazal’s way of making sure they’d all get laid a minimum of once a week?