Marriage is an entirely human-designed legal and fiscal union, we can socially and legally define it however we want

Because of the need of consensus/popular vote, we simply need the majority to agree that LGBTQ+ people exist and deserve equal rights (which they absolutely do and the majority of people agree) and by consensus social design of marriage - their desired unions are 'marriages'.

Where religions insist that marriage is only between a man and a woman, they are a minority sect who do not socially define marriage for the population and should rightly be ignored.