You miss the point that Catholics see in this scripture. There was a ceremonial jewish punishment for refusing to give your brother an heir that applied in this OT passage. God Himself gave an extra punishment (death), instead of allowing just the regular punishment, and Catholics take it to be because he spilled his seed and not because he refused to give his brother an heir.
I am hopeful that Catholics and Protestants don’t necessarily need to have identical lists of sins to come to similar conclusions on right and wrong. I am hopeful that the general directive to “go forth and multiply” would inspire both denominations of Christianity. Unfortunately, in todays culture, that does not seem to be the case.
RE: I am hopeful that the general directive to “go forth and multiply” would inspire both denominations of Christianity.
I am hopeful too, but the original question you posed still needs to be answered: If you DO NOT multiply, is it as egregious a sin as committing fornication?
My reading of scripture tells me the answer is “no”.