I still don't get why in America it's popular to say that ones marriage is sexless. A study came out a few years ago trying to prove that married people were happier, but only ended up proving that on average married people have more sex.
I don't get why the joke would be funny, I would find it sad if true and bizarre if made up.
I suspect it is a misinterpretation of another fact. Very often, a couple has less sex after marriage than before. This leads married people to believe that marriage reduces sex.
It's just playing off the old "ball-n-chain"-style of marriage humor often trotted out by stand-up comedians and sit-coms for a laugh. I wouldn't take it too seriously.
married vs. unmarried. And keep in mind this is the average. I'm sure you can find Warren Beaty-style hounds that married couples literally wouldn't have enough waking hours to keep up with.
I don't get why the joke would be funny, I would find it sad if true and bizarre if made up.