We had this!! I got 2 young females as we didn't want guinea pig babies either, turns out both were girls but one was already with children!
She had 3 healthy babies, we took her to the vet when we realised she was pregnant, like you both pigs were growing but one grew 'saddle bags' and you could feel the babies by the time we got to the vets she was almost ready to give birth and did so about a week later.
You can get males neutered but its dangerous, we got the only male baby neutered when he was about 6 months old but he chewed his own stitches and died.