Dear Matt —
The Church has rules and norms about scandal and occasions of sin. You should not live together because it gives scandal and is an occasion of sin. It:
- hurts both your reputations
- risks putting a child in an unstable and dangerous situation, and
- jeopardizes the stability of a possible future marriage.
If you both are ready to assume the responsibilities of marriage, you should have enough money between you to rent a room for her, in someone else's home or apartment, until the wedding date, which should be fairly soon. You could also get a third roommate, another woman, to be a sort of chaperone, though that solution is imperfect.
- Why is she not working and earning enough for her own apartment or room?
In extreme situations of survival or necessity, it is moral to live together, if scandal is avoided and precautions, such as the chaperone, are taken. Otherwise, it is imprudent to live together before marriage, because it is a near occasion of sin. Most people would find it impossible to remain chaste.
Living together is not an impediment to marriage, but fornication would require that you go to Confession before receiving the Sacrament of Matrimony. Most Catholic marriage prep programs would ask that you separate, or, if that is impossible, that you remain abstinent.