Sex before marriage to conceive

I know sex before marriage is a sin, but is it a sin if you KNOW you’re going to marry the person, and you have sex to try for a baby who you will teach about Jesus? And if your family will spread God’s love? Boyfriends parents will only pay for his half of the wedding (and demand it be the way they like) if he waits four whole more years to get married and will cut him off if he doesn’t wait, but we want to start trying for a baby now. We just want a little one to spread the faith to, a family of God. So is it a sin to have sex before marriage if you’re trying to conceive to give a faithful baby to the Lord and spread the word of God as a family?