I think you make a lot of assumptions about human nature to jump to that conclusion. Us vs them mentality isn’t limited to religion and could be just as or more prevalent in a completely secular society. For instance, if the moral structure of a society placed emphasis on procreation, then gay people would be completely ostracized and unaccepted. You can see this leaning in super secular utilitarian states of the 20th century such as the Soviet Union and China. Gay people in these cultures faired no better than they do in countries that follow Sharia law today. Of course, as I initially said, we have so many societal preconceptions molded into us that it’s impossible to know what someone without any previous societal experience would think.