[soVeryTired]

The double negative confuses me. Are they saying young women never do that or they always do it?

[pngai]

I could be confused too, but I would rewrite it as

"It is a universal practice for a young woman to fart in her husband's lap."

[SomeStupidPoint]

I think 'experience' might be better than 'practice', in the sense that it's an accidental fart.

Which makes it a strangely comprehensible joke -- "there's never been a wife that didn't fart in her husband's lap", which I can see being said as a teasing, crude joke. (Especially if that "girl's don't fart" thing is an older pseudo-belief than I had thought.)