Pair bonding.

Nails in the Religious Coffin: Sex, Drugs, and Contraception - Beliefnet

The thing that religions generally do not recognize is that reproduction in humans is not just popping a litter out and seeing who survives. Reproduction in humans is a long term investment if the zygote is going to get to puberty. One of the main evolutionary purposes of pleasure in sex and the evolutionary reason for the hidden estrus in humans is that the pair bond is essential for reproductive success, that is getting a reproducing offspring into the world. Religions generally accept the fact that sex after the first rape will result in a pair bond, which is one of the reasons they insist on marriage prior to sex. But this pair bond is dysfunctional for the church since the loyalties of the pair will be to the family rather than to the church. Therefore the restrictions on eg contraceptive sex which might make the pair bond more important than the church.