If you want to understand why a group believes something, ASK THAT GROUP (/r/OpenChristians). Their opponents are committing outright slander in this thread. A humble, loving Christian would never give most of the top-rated answers you're getting. Their hearts are unloving and uncharitable to their fellow believer, arrogant and boastful of their own rightness.
Now, since I am an affirming Christian, I can give a real answer to your actual question.
Some Christians tend to pick up the Bible and read it as if it's a book of rules written in English. "Look, this verse here says ABC, therefore you should clearly do XYZ." That seems to be the approach to the text that you're taking with your original post, which is why you're coming to a conclusion many other people don't come to: they're not using the Bible the way you're assuming it should be used.
Now, why do many Christians not use the Bible as a plain-English rulebook?
For one, the Bible is not written in English. It's written in Greek. (Original OT texts are in mostly Hebrew, but the oldest complete texts we have are a Greek translation, there's no real dispute that the meaning of the Greek OT is different from the Hebrew OT for our immediate purposes, and the relevant texts are mostly New Testament anyway.) Translation is not exact, just by its nature. There are often multiple ways a single word could be used, and more than one of them might make sense. Sometimes we don't even know what a word means! So we should always be asking ourselves, "did the translator get this right? How else could it have been translated? Are there alternate translations that just aren't being presented? How much of this translation is certain, and how much is guessing?"
(Though it's not too applicable to our current topic, we should also always be asking ourselves about textual variants. Our ancient manuscripts often don't agree about the text in the original languages! They're ~99% identical, but that ~1% is still quite a lot of text. So to produce that English book you're quoting, we first have to collect all the ancient manuscripts we have, identify their thousands of points of disagreement, and figure out which variants are most likely to represent the original text, to compile a critical text that can then be translated.)
(For this reason, it's worth pointing out that by any modern standard, the King James is just a bad translation. I'm sure they did the best they could, but in the intervening centuries we've found better texts to work from, we've gotten better at translating Greek to English, and the English we speak is just different now. NRSV is now the standard for academic discussion, though many other translations provide significant value. I tend to use classic.net.bible.org because it provides deep translation notes, shows multiple translations in parallel with the original language texts, and also because the New English Translation itself works well for me.)
(It's also reasonable to question which books should even be included in the Bible, but I axiomatically assume "these 66 books count, period." Otherwise, where do we even have to stand to have a conversation? To my knowledge, there aren't any books rejected by the protestant canon that would affect the current discussion.)
So you see that before you even pick up that English translation of the Bible and read it, you're relying on the work of layers upon layers of experts making their best guess about a lot of things, plus a good bit of Church tradition to even define what books are included. That means what you have in your hands isn't the Bible; it's an approximation of the Bible. It's still the most valuable text you're ever going to have, but we must approach it with humility and uncertainty, because those experts could be wrong. And if they can be wrong about textual criticism and translation, we can just as easily be wrong about understanding.
Now we can finally approach the question of how this text should be used. Because it's not really a book, it's a library, containing numerous documents, each of which has its own intended audience and purpose. Not a single one is a book with the intended audience of "all Christians who will ever live anywhere" and a purpose of "Here are the rules for how those Christians should live." Not only is the Bible not English, nor is it a Christian rule book, it doesn't even contain a Christian rule book!
Yet we also assume that by understanding the Bible we can understand how we should live. So the next question is, how do we determine how to live by humbly approaching this approximation of the text? And now you have the important, fascinating, and neglected field of Christian Ethics. For that, I'm going to point you to David Gushee, who wrote the book on the subject. In fact, I recommend reading basically everything he's ever written.
https://www.amazon.com/Kingdom-Ethics-Following-Contemporary-Context/dp/0802876110/
So from this perspective, I think your question becomes, "How does an affirming Christian understand this list of texts?" This post is getting too long, so I'll reply with that shortly.