My dad's dog is 14 years old and still super energetic-- you have to hold her back on walks, she'd just keep going for miles on her stubby little legs, and she's spayed (never had puppies, either).
All the stuff about getting lazy once they're fixed, or having better temperaments if they've had one litter, is just superstition. Like Beth said, there's scientific evidence that it's not true, but people just go on believing it anyway (kind of like your Christian friend you mentioned the other day-- just believing what he wants, against all the scientific evidence-- see what I'm gettin at?
).