Forgiveness is available to all who repent, but mercy does not rob justice. If you commit sexual sin, there is a price to be paid before forgiveness is gained, and losing the privilege of receiving the Cahtolic sacrament of marriage would be part of that price.
"Cheap" grace is not part of the Christian equation, though many seem to think it is. In God's vocabulary, "I'm sorry" is a verb.
Well gee... So much for repentance and acceptance from God.
I'm not sure what sort of Catholicism you're representing, but there's no claim or requirement in Church law that I'm aware of that a couple who gets pregnant before the wedding "loses the privilege of receiving the Catholic sacrament of marriage". In fact, that would be silly: equivalent to excommunication.
Many priests won't perform such a wedding until some time after the baby is born, though, because they think marital consent is not very freely given when a pregnancy is involved, thus rendering the marriage's validity suspect. It has nothing to do with anyone's sin, and everything to do with the degree to which people are, or are not, acting freely in that situation.