Nope, it's not against the rules (though it's not technically fair).
Of course it's fair! It's within the rules of the game. The object is to advance from batting at home plate to first base, then second base, then third base, and finally home again without being tagged out by the other team or forced out. Stealing a base is a perfectly acceptable method of advancing that much closer to scoring. In fact, one of the most exciting plays in baseball (which is, admittedly, exciting only in brief spurts) is a runner on third stealing home.
(A force out occurs when a runner does not have an unoccupied base behind him and the ball is put into play so that the batter can advance to first - not a ball hit in the air and caught - and the defensive player holding the ball touches the base to which the runner is advancing before the runner does.)
The only problem with Ralph leaving the base early would be in slow-pitch softball. In that version of the game, the ball is tossed so softly by the pitcher to the catcher that someone who limps when he or she runs could steal second. But it looks and sounds like they weren't playing softball at all.
If Clark went to college on a football scholarship and didn't kill anyone playing a collision sport (basketball is a contact sport, football is a collision sport), then why does he think he has to act like a hopeless idiot on the diamond? He doesn't smash every coffee mug he picks up, so why would he send a baseball into orbit just because he hit it? I'm not sure the bat OR the ball would survive that impact, unless Clark projected his aura around them when he swung.
Apart from that, I thought Lois' attitude toward Ralph was understandable, if a little over the top for a company game. Maybe there's a little back story there where Ralph accused Lois of getting too much Superman help with her stories lately? Or Ralph making cracks about how much the Mad Dog has mellowed since she married?
But her attitude toward Clark was puzzling. No sex unless he took her side against Ralph? Granted, Ralph isn't the easiest guy to like (amazing how much mileage we get out of a character who had so little screen time), but there's no kung-fu fighting in baseball (use Tom Hanks voice)! And she's mad because he wouldn't let her clobber Ralph? Not to mention her look after Clark hit the winning walk-off grand slam. Her team won, she beat Ralph, so why isn't she happy with her husband?
This was a cute story and I really did like the baseball imagery. Thanks for the ride.
A true story: A minor league radio announcer once described a fly ball pursued by a right fielder in this way. "He's going back - back - he's on the warning track - his head hit the wall! Now it's bouncing back toward the infield! The center fielder picks it up and throws it to second!"
I guess baseball can be pretty violent after all.