This goes back much farther than I Love Lucy. Specifically, it goes back to the Hays Code that reined in the movies starting in 1932.
The intended effect of the Hays Code was to cut down on the overt display of sexuality evident in the early years of sound movies. (It had numerous other clauses as well, but they’re not as relevant.) Movies couldn’t show skin, or people in bed, or even much physical contact between the sexes.
Then as now, however, nobody objected in the same way to violence. Today, certain sexual scenes will get an NC-17 rating while violence against women rates only an R. (Check out the documentary This Movie Is Not Yet Rated.)
The screenwriters knew this instinctively. When they developed the screwball comedy genre they deliberately sublimated sex to violence. The couples that everybody knew were meant for one another in the end battled their way through the entire picture, yelling, screaming, spanking, hitting, kicking, spitting, pushing, tussling.
Audiences understood the code perfectly well. The louder and more physical the interaction was in the early part of the movie, the more passionate they could imagine the sex that would occur after the credits rolled.
Lucy and Desi were squarely in this tradition. They were a married couple but had to sleep in separate beds and couldn’t use the word “pregnant” no matter how large Lucy’s belly grew. So how to show how much they loved each other? By using language that everyone in the country had seen continually for the past twenty years. Desi’s spankings of Lucy were the equivalent of him fucking her. If Lucy couldn’t sit down afterward, it was because she was so sore from the pounding of his massive cock. Every adult understood this, and the kids just saw the comedy of an adult being treated like a child. Win-win.
You still see vestiges of this past today in screwball comedies, but that’s mostly homage because there’s no good reason not to have the couples together from the beginning. Violence as sublimated sex began to go away in films of the 60s, as soon as the Code was jettisoned. It never will entirely disappear, because the pictorial symbolism is too good to pass up, but its use has changed drastically.
This isn’t my theory, BTW. Read any good book about 30s screwball comedies and you’ll find this.