Goldfish don't per say 'mate'. They do not pick mates either. They are an egg scattering breeder. Males will bump and chase a gravid female to encourage egg release then fertilize the scattered eggs. Then both males and females are just as likely to consume the eggs, too. Females can also release eggs without a male present. Bumping and chasing behaviors may be displayed by dominant males even if a female is not present as they will treat other males in the same fashion but it usually indicates a tank issue (toxins present, too small of tank, overstocked tank). Please ask if you have questions!