Homosexuality and religion
There is the view that sexual activity outside marriage is bad. There is also the view that sexual activity that is not about procreation (making babies) is bad. Many Abrahamic religions (Judaism, Christianity, and Islam, mostly) still have this view. For them, homosexuality and bisexuality are unnatural, and wrong. Christianity calls them a sin.  Many Islamic countries punish such relations. These groups see marriage as the union of a man and a woman.  Marriage mostly has the purpose to have children. These people say that because with a same-sex marriage, people cannot have children without help, same sex-marriages are bad,  Most forms of Christianity have taken this approach.