“Tolerance” and “diversity” are the buzzwords today, and those values are defended viciously. As such, many people may be reluctant to admit to the fact that religious differences can be a serious dealbreaker.
Religion heavily influences values and morals. Japan, for example, lacks the serious taboo against suicide that is present throughout the Western world. Why? Suicide is a sin in Christianity which is the main influence on Western culture, but not in Shintoism which was Japan’s earliest religion. Religious influences, as such, can lead to differing value systems that do not mesh well in a relationship.
Contrary to what some people may think, only wanting to date someone of the same religion does not mean that a person is a bigot. Many people want to be able to share their spiritual journey with their partner and that does not always work across religions. A Buddhist and Muslim, for example, may both have plans to grow in their faith, but those plans may not be compatible. The two can still be good friends, but they may decide against dating someone with whom they will be unable to share the spiritual part of their life. There are also, of course, practical concerns when the purity codes of different religions do not mix. Cooking two dinners or owning two sets of cookware could get very tiring, very quickly.