Whenever I read 1Cor. 7, I feel St. Paul is not implying that the main point of marriage is to give birth to children, but rather a mutual union of man and woman, especially in sexual matters.
In the Old Testament, it seemed that marriage was a union for giving birth, but as we learn from the NT, you don't have to be a biological father to be a father. This changes the framework of marriage, for although we might imitate Christ's relationship with the Church, we are also considered "sons" by adoption.
In the Old, it was be fruitful and multiply in a literal sense. Now, we should be fruitful and multiply in an evangelical sense. The Old cared about descendants of Jacob. The New cares about adoptions into the Church. This definitely should change the framework of marriage.
I have a feeling the Holy Fathers thought such about contraception not because of marriage relationships, but because they felt the seed was a human being, a full human person. Ejaculation to them was abortion.