My girlfriend was diagnosed with pcos and doesn't have her period unless she's on the pill. does this mean we can't eventually have children?

Be optimistic: I've known women with pcos, which is fairly common, who've complied with therapies and become mothers. In the meantime, the finest thing you can do for the woman you love is to find some form of physical exercise that's fairly strenuous and that you enjoy doing together. Be strong, be there for here, and the joy of fitness may well lead to the joy of being parents.

Not at all: No, it just means it's pretty likely that you and your gf may need some medical help with conception.