Instead of blaming gays and lesbians for declining birth rates, people should ask why heterosexual couples are having fewer kids. So many heterosexual couples are now having either one child or none at all. People are marrying later and having fewer kids. Responsible people are putting off marriage and kids until they can afford to take care of them, and in these days of high rent and often high student debt, it often means people are waiting until their careers are well established before they have kids. That can take to their late 20s or their 30s, while people used to have kids in their early 20s. (Irresponsible people, of course, are still having lots of kids without bothering to worry whether they can provide for them or not.)
Heterosexuality is never going to become unpopular. Heterosexuality is doing extremely well. The vast majority of people prefer heterosexuality and have no interest in changing. Declining birth rates aren't a result of people "turning gay".
-k