No Biblical basis for this and just my gut feeling but my thought is that the books of Acts and Luke seem to contain so much detailed information that although it appears Luke was writing to an individual named Theophilus it would be expected that the information contained would likely be shared...
This is most certainly true. But, we still have a tendency to want to do things our own way even with this knowledge.
And this is where the answer is found. They weren't coerced or forced by God to obey in the end, they did it out of love and respect for God.
What is the basis used to identify God's phrase, "Be fruitful and multiply" as a commandment and not just a blessing? Sort of like when we wish someone to do well we might say, "Break a leg." That's certainly not an order.
I have no problem embracing the concept of Trinity. I just resign it to the fact that is God and not a human person. God, who can do all things and is all things. It is not beyond my reasoning to understand that God can be Father, Son, and Holy Spirit. After all, He is God. The only God.