Please do a search on the site for all of the reasons why you shouldn't breed.
You seem to think that because you'll keep the babies, you won't be contributing to the overpopulation problem. Wrong. For every pig you breed to keep, there's a shelter pig that gets put down because you could have adopted instead of bred, but you didn't.
Also, the health of the sow needs to be called into question. What age is she? Has she been bred before? There are many dangers to her health that can be taken completely out of the question if you don't breed. Please read this page:
Guinea Lynx :: Breeding
What happens if your sow has a huge litter? Are you prepared to care for 9+ guinea pigs in an appropriate manner? How do you know your friends will be responsible? You changed your tune as you wrote the post: you'll keep them all, but by the end you'll give them away.
You've already done the worst thing which is the actual breeding. I hope you do the right thing by not breeding again, and taking care of them all as you say you will. But why should people get mad? Just do that search and you'll see.