It may be possible if your are lucky, but don't count on it.
As already answered by Zach, airlines very well do know who should sit where. The question is if they always check, and in which detail.
Case in point: A budget airline I travelled with recently has different "classes" even within economy. Specifically, bringing checked baggage and getting a snack are extra. We booked late and got seats in different rows. After boarding was complete, I noticed the seat next to my wife was empty, so apparently the person who had booked it didn't show up. I changed to there in order to sit next to my partner. 20 minutes later, stewardesses go through the cabin with a paper and hand out snacks to selected passengers. We booked without (weekend trip, hand luggage was enough), but I received a snack, most likely because I was sitting in a seat that had booked the higher package. They didn't check boarding passes or anything, just seat numbers.
So if you manage to sneak into a seat that was book but ends up empty, you may have a reasonable chance of not being noticed. The crew will see the seat is booked, see that someone is sitting there, and if the gender matches, will probably not question you.