Because some ancient sex manuals written by ancient goat herders said that gay sex is gross, but it also said love thy neighbor, and none of the peasants at that time had the wherewithal to think "hey maybe only one of those things actually makes sense"

Edit: I'm not giving the peasants enough credit - a bunch of them probably did have the wherewithal, but the people in power murdered them for questioning the ancient sex manual, so that stopped that pretty quick