This is a really good topic to chew on and discuss. Here are my thoughts on it, and particularly on questions you asked or things you said, SeoulSearch.
-Do your best not to pay attention to what people on the internet say. 90% of people on the internet are idiots 90% of the time. I would not take spiritual advice from someone online who didn't know me, or really care much for what they have to say, to be completely honest. It really doesn't matter to me that some will say online/emotional affairs don't "count" or that some will say this or that. If we are striving to operate according to what the Bible teaches, then we know we are on the right track.
-I think the way that we look at divorce, marriage, and sex is so often off-base with how it is treated in the Bible. For example, if you look at the way the Bible speaks about marriage and sex throughout, they are nearly synonymous. We were created in such a way that sex was intended to be a sort of seal for the marriage. There is a ceremonious aspect of marriage where commitments are made before God, friends, and family, and then God gives us this means of, privately and intimately, making two into one. So when we develop intimacy with anyone, whether it's physical or emotional, and they are not someone we have covenantally committed to, we are putting ourselves through a divorce experience. It messes with us in a major way. So when we think about divorce, we can't just think about it from a legal standpoint, we have to think about it from a sexual standpoint as well. And just as a clarifying point, that is not to say that if you have had sex, that means you were married because there are other things that make a marriage a marriage, but at the heart level it is as if you were. In a sense you could say you might as well have exchanged rings with the person you slept with and then given them back to each other whenever you decided your relationship with them was over.
-Regardless of the practice of any church, all sin is the same and should be treated as such in the sense that if we confess ourselves to be sinners before God and trust in Jesus Christ for our salvation, we can know that we are forgiven. There are a ton of scenarios that you could think through regarding when it is or isn't right to re-marry, but the bottom line always comes back to the heart--is your goal to honor and serve the Lord or are you just looking to serve your own desires and trying to find loopholes to do that? And along with that, you can only control what you can control. If someone has a spouse who divorces them for an unbiblical reason, that decision isn't on them, it's on the spouse who left. If a church is dogging someone just because they got divorced with no mind for the reason or circumstances behind it, then that is not a church worth being a part of.
-In the end, the Bible isn't about the letter of the law so much as it is about the heart. If a person is having sex over and over again, they are sinning. If they are having sex over and over again presuming that they are already covered by God's grace, I would say the Bible argues that they aren't even a Christian. They can't just ask God's forgiveness any old time they want in that scenario.
-It is true that there can be a scenario where Person A has had sex with a bunch of people and is still biblically eligible to get married while Person B may be divorced and can no longer biblically marry, but there are a couple of caveats to this:
*Person A needs to have repented of their sin, meaning they have not only asked forgiveness but have turned away from that sin in their life.
*It is also worth mentioning that even then, there is a good chance that he/she is going to have to work through the consequences of their past sin as they come up in a future marriage. Sexual sin especially has a way of following us around even a long time after we have repented.
*Person B in this scenario is someone who made a decision to divorce their spouse for an unbiblical reason, knowing that they did so for an unbiblical reason. Even if they were later repentant, to me the only way to honor their repentance would be to remain unmarried (unless they could somehow restore their previous marriage, in which case they could remarry).
-A big problem with what I see in the experiences you described, SeoulSearch, is that there are individual Christians or churches trying to "enforce the rules" without really any mind for the heart or spiritual well-being of the person involved. It should always be about being faithful to what Scripture says, but part of that faithfulness is loving and shepherding a person well in community. That doesn't mean giving the person what they want, which a lot of churches also want to do, but about examining the heart and a person's willingness to honestly examine themselves.
-Being single/unmarried does not have to be synonymous with being alone. We may not be able to develop quite the same type of intimacy with another person as we would in marriage, but that doesn't mean that we have to be alone. I am 29 years old and single, and for a time I really struggled with depression and thoughts that I am alone and no one really cares. I even had passive suicidal thoughts of just really not wanting to be alive and not wanting to fight my demons anymore. It was sort of like...I'm not going to actively hurt myself, but if something happened to me that ended my life, I wouldn't mind that. As I had those thoughts of being alone, I would get this other voice in my head telling me, "What do you think [so-and-so] would think about that if you said that to him?" There was one guy in particular who came to my mind who has been a really great friend to me over the past few years. But there were a few others that came to mind as well. And I shared my passive suicidal thoughts (because I wanted to share them out loud with someone so they weren't just in my head) with another friend/mentor of mine who told me how it would really hurt him if I were gone. I have been blessed with an amazing church family who has really lifted me up as well over the past few years. So regardless of what any single person's status is--never married, widowed, or divorced--and whether or not a future marriage in good conscience is a possibility, being alone is not an inevitability. It is only an inevitability if you decide that it is.