The J man most definitely does not forgive pedos and certain other blasphemies. Read Matthew again and you'll learn what he thought you should do to them. It wasn't forgive them, moreso Kill Them.
We can't use the law to judge our brother otherwise we end up like the Pharisee, and then just like the second son in the story of the prodigal son, Christ will turn to us and ask 'why could you not forgive your brother?' Killing people for going against the law when the very same scripture decries placing the law over the condition of your heart is not the way that leads to salvation.
God is a literal ocean of forgiveness and the definition of everything good, every single one of our collective sins are like specs of sand that just get washed away by this ocean, and there is not a single sin that cannot be forgiven through repentance. God's anger and wrath would have already fallen on every single person that the law deems worth of death long ago if this was the case. The reason that it hasn't is because, as the church rightly tells us, the condition of your heart always trumps the secondary aspects of the law. It's even present in the first two commandments: thou shalt love the lord your God with all your heart, all your mind and all your soul, and that thou shalt love thy neighbour as thyself. We cannot love our neighbour if we aren't willing to show them the same infinite forgiveness and patience that the Lord shows to all people on this earth.
The /only/ sin that cannot be forgiven is the sin you do not repent of, and all repentance is when you really boil it down is a re-orientation of your life and a sincere desire to change who you are, or correct at least some of the faults that you have. The church fixes the point of orientation as Christ, as this is part of the process of Theosis, or literally, 'becoming as Christ is', but for the sake of Mushoten (because it doesn't directly reference Christ) the themes of repentance and forgiveness are illustrated in Rudeus' constant mistakes and his attempts., however flawed, at fixing those mistakes.
Having a moment of weakness and cheating due to grief rendering you irrational would be bad enough. Deciding to run with it and flat out take on a second wife is absolutely fucked and the fact that it's portrayed as cool and good relegates this series to, at best, a guilty pleasure that you enjoy simply for the sheer spectacle. A degenerate is reincarnated to a degenerate father and grows up to be a more successful degenerate than he was in his previous life. That's his character arc.
It's not played as cool in the slightest, Norn literally flips out as a result of it and Rudeus himself has extreme misgivings about the whole thing which is expressed by the guilt he feels when Norn berates him for what he's done. Sylphy is the one who forgives both Rudeus and Roxy for the acts they committed. Rudeus has these misgivings for the rest of his life, which are apparent to anyone who's sat down and actually read the webnovel.
The thing you're forgetting is that the world this story is set in is fictional and that only the Millis faith believes in monogamy. The rest of the world has absolutely no problem with this, so of course Sylphy would have no problems with Rudeus taking a second wife. If you have a personal issue with the author creating works like this because it doesn't suit your palette, then alright, but moralizing about all this makes no sense in the context of the story because it just comes across that you don't actually understand or care about the world or the characters inside it even if that isn't the case.
This might sound a bit weird, but bear with me a little: when talking about different kinds of people, Saint Paisios of Mt. Athos gives the example of honey bees and carrion flies; when a carrion fly enters a room filled with flowers and sweets, it ignores everything sweet and immediately seeks out the rotten meat in the corner. When a honey bee enters a room full of rotten meat, it ignores all of the rot and immediately looks for the small flower that's growing through a neglected crack in the floor. The carrion flies are the people who only ever see the bad when presented with a wall of good, the honey bees are the people who only ever see the good when presented with a wall of bad. For our own health, the Saint advises that we all try to be like the honey bee; seek to justify the good in others and not condemn them for their faults.
