Where is the dogmatic teaching that cohabitation before marriage is always wrong?
The main objection to cohabitation is that it almost invariably involves fornication, which is an intrinsic moral evil (CCC 2353), meaning that it is always and everywhere gravely immoral. Conjugal love is meant to express the marital covenant between a husband and wife and is meant to be a mutual giving that is truly free, unreserved, faithful, and fruitful.
The possibility of procreation is a further reminder why married sex is called the “marital act”: because it is ideal for a child to be raised in a stable, two-parent household. This is not to say that single parents aren’t good parents but rather that the difficulties accompanying single-parent households are a reminder that couples should prepare for marriage well, i.e., in a virtuous, self-giving manner so as to lay the best foundation for a lifelong commitment. (To be clear, one may prepare well for marriage and still be subjected to a spouse who, God forbid, abandons or abuses the family.)
The Church’s infallible teaching on fornication is rooted in Scripture, as both Jesus (Matt. 15:19-20) and St. Paul (1 Cor. 6:9-10, Gal. 5:19-21) make clear the immorality of this misbehavior. For more on this topic, see this Catholic Answers presentation.
By Tom Nash