“Newer” religions, using the term loosely, don’t have that many followers. One way to have more followers is to encourage your existing followers to have more children. That’s why birth control, abortion, and such are banned or discouraged by some religions. Homosexuality is the same thing: a gay couple isn’t going to “be fruitful and multiply”, they’ll just “be”. So if you ban homosexuality, maybe you’ll get more children and your religion will grow.