Whenever there’s a broader discussion about LGBT rights, somebody invariably brings up the “argument” that “if everyone were gay, humanity would end because nobody would have children anymore.”
That’s a classic non-argument meant to invoke fear by presenting an impossible scenario as an obvious consequence of whatever the speaker is against (in this case, equal rights and treatment for LGBT people and couples). It sidesteps the actual issue, which is that many people are treated badly, sometimes inhumanely, because of the gender of those they’re attracted to or how they present themselves (like not adhering to styling or behavioral conventions for their assigned sex).
Still, to address it: sexual orientation naturally varies from person to person. Demonizing that diversity won’t erase it; it will only drive people to hide it. And even if somehow the majority of the population ceased to be straight, humanity would still not end. A lot of people want to and do have children, regardless of their sexual orientation.