The death rate for guinea pig sows and pups is high, in no small part because the pups are huge in relation to the sow, and the death rate in childbirth is high.
Guinea pigs are susceptible to a number of genetic diseases that can cause lifelong problems and abnormalities. Satins can have osteodystrophy, a very painful bone disease. Lethal whites are blind, deaf, with wonky teeth and immature digestive systems. And those are only two of the problems that can occur.
Do you know the characteristics of pigs who can produce those abnormalities? And can you say for sure that your pigs don't carry those abnormal genes? And if your pigs do produce pups with either of those characteristics, do you have the thousands of dollars it will take for their vet bills? And the patience to care for blind, deaf pigs their entire lives?
Guinea pig sows don't "choose" when to mate. Sows are receptive to males when they're at a certain point in their reproductive cycle, but it's not a choice. It's totally controlled by hormones.
There are thousands of unwanted guinea pigs in rescues and shelters that need good homes, and there's no need to ever deliberately breed another guinea pig. There are enough accidental pregnancies to keep the species going forever.
Just so you're aware, this forum is strongly anti-breeding, and is both pro-adoption and pro-rescue. You may participate in the forum if you choose to breed your guinea pigs, but you may not display pictures, nor ask for or offer any information on breeding.