Wrong. Emphatically wrong.
One missexed pair from a petstore resulted in over 20 guinea pigs in one year for this family:
20 guinea pigs in Danbury, Connecticut
One year is not a very long time in which to end up with twenty or more unplanned-for pets. That's a lot of animals for which to find good homes, and most rescuers cannot take large groups in on a regular basis.
Guinea pigs are physically capable of reproduction at the age of 21 days. Their gestation period is 59 to 72 days (a little over two months), and they have litters of usually between 2 and 5 pups, although more are certainly possible.
Guinea pigs give birth to precocious young, which means their eyes are open at birth, they are fully furred, and begin walking, running, and eating adult foods almost immediately. In 21 days from right then, they are capable of making more.
PLEASE make sure your pigs are of the same sex before housing them together, unless of course the female(s) are spayed or the male(s) neutered. Don't take the chance of having accidents! Even "a little supervised playtime" can result in one or more unwanted pregnancies.
BOTH sexes have nipples, and not all males have definitive testicles, so a casual glance at a guinea pig's underside will NOT give you a clear answer.
A cavy-knowledgeable veterinarian can properly and definitively sex your pig for you. Other methods can be found on the Cavy Spirit website:
The FACTS of Guinea Pig Life
and on Guinea Lynx Forums posts:
Ridge method 1
Ridge method 2