How does banning same-sex marriage adversely impact the rate of birth amongst heterosexuals? And if banning marriages that can't result in babies were a way to

How does banning same-sex marriage adversely impact the rate of birth amongst heterosexuals? And if banning marriages that can't result in babies were a way to increase birthrate, then why are heterosexual people who cannot have babies (post-menopausal women, men who've had vasectomies) allowed to get married?