The Church, historically and today, offers much less grace when it comes to rules of sex and marriage. The vast majority of churches teach that sex is allowable only between a man and woman in a Christian marriage. But if the Bible isn’t clear about this, where did the idea come from?
The early Church cultivated a strong anti-sex viewpoint. Some leaders wrote that total sexual abstinence was required of all Christians. Others taught that sex was meant for procreation only. On a more positive side, the early Church also helped end the common practice of sex with early adolescent boys and girls (whether slaves or prostitutes). Christian men were also told to stop having sex with slaves and prostitutes, which was a radical idea at that time.
Church leadership, then as now, was a difficult job. Even if we try to strongly incline toward grace, an organization like a church can’t be a “free for all.” If the Pastor says he feels the Holy Spirit telling him to take in a concubine in addition to his wife, the church Elders will need to pass judgement on those shenanigans. Someone needs to decide who’s allowed to teach the children and what to teach them. Christian congregations still exist that teach sex for procreation only—though there are very few as men tend to be pastors—and congregations also exist that share my freethinker views.
The vast span of beliefs held by Christian churches gives Christians an opportunity to worship in a place that shares their beliefs about sex. I don’t imagine most folks spend much time struggling through why they believe what they believe about sex, but raising teenagers tends to get those questions frothing.