I am concerned with them not getting along with yelling and slamming doors… I know they are young only 21 years old but they will be met with difficulties as taking care of their own home and having children (pregnancy) to take care of financially will be stressful
They need a real relationship with God thru Jesus...
Not externally imposed religion...
If they profess faith, they need mature Christian friends or Elder Christians to help them think through what is going on.
Most people need discipleship/ counseling / deliverance before a successful relationship with the Lord.
Earthly relationships follow, after the First with God.
God's fruit of the spirit is love, joy, peace, patience, goodness kindness, faithfulness, self control...
Sounds like they are lacking it.
I personally don't have children (yet) but I wouldn't allow / support her to be "comfortable" in a spiritually harmful situation.
I would communicate it out of deep love and concern for her soul ... no dating losers...
and we need to trust God here, and focus on the eternal and pray for God's will in God's timing..
If she wants to act like a teenager I would treat her like one...
No judgment with your parenting however, I wish your pastor would have been more clear about what the word of God says about their relationship and the call to Purity and to flee every type of immorality which would include any sexual contact before marriage and I wish your pastor would explain how marriage is God's best and what He wants and expects