Why can't gay people marry each other in some countries?

In order for same-sex marriage to be legalized in a country, the government must enact a law or amend the constitution to permit it. Alternatively, a high court can order the government to permit same-sex couples to marry.

If a country has not legalized same-sex marriage, then it is because this has not been accomplished. The reasons could be cultural or religious. Some cultures and many religions actively discriminate against gays and lesbians.