The first thing is what do we mean by "sin"? Most people (both inside and outside of Christianity) think of sin as "doing bad stuff" or "breaking rules" or "refusing to submit to the arbitrary authority of some paternalistic, sexist, cartoon diety that doesn't really exist for the purpose of keeping people in line and denying us the ability to flourish as the beautiful, wonderful, amazing people we are." Yes? So then, we say, of course we don't believe in "original sin". Babies are beautiful, wonderful, and not subject to the same sort of BS as the rest of us. Sure, all the rest of us get caught up in rule breaking but not the 10 minute old baby. How could they? Not only do they not know the rules, they don't have the mental development for such a conversation about breaking them to even make sense. And if sin were primarily about rule breaking, I'd tend to agree with you.
So we go to Adam and Eve, the original sin originals. God said "Don't eat this here apple" and passed a law. No matter how arbitrary it was, those humans should have followed it. He could have said, "Always wear a hat on Tuesdays" and they should have done that too. And why not? He basically told them "Live in this paradise, don't work, be naked all the time, and have lots of sex". Couldn't they have done this teensy weensy thing that God asked of them? But no, they couldn't and so they screwed it up for all eternity. And now because of this "original sin" the rest of us have to live in smog filled cites, work for the Man, cover our naughty bits, and (except for the 1960's) keep our sex lives quiet and to ourselves. That better have been one damn good apple, guys.
But that's not really what this story is all about, and it's not what original sin is all about. Because sin is not really about rule breaking at all. It's about being "curved in on ourselves" (incurvatus in se if you want to get all geekly Latin about it). Its not first and foremost about our actions, its about our primary view of the world. Which, most of the time, is our own belly buttons. The nasty stuff we do to one another (and ourselves) is the result of our own inwardness, our own self-centered world that really doesn't have much room for our neighbor or for God.
St. Augstine wrote (somewhere) about the deeper meaning of original sin with something along the lines of "We don't want God to be God. We want to be God." God created humans to be in relationship. Relationship with God, with each other, with all that is. God created us whole, where our will and God's will for us were one and the same. But God also created us with free will, we're not puppets, and we have the ability to choose to reject God. And we do. Again and again and again. Every day. Even when we are trying really hard not to (maybe especially when we are trying really hard not to). Even Jesus prayed "Not my will, but yours be done." Adam's sin was not doing something God told him not to do, but in putting himself above God, saying in effect "I don't need you to be God. I'm perfectly happy being God, thank you very much."
When I talk about sin with people who don't know our churchy jingo (and even with people who do) I like to use the word "brokenness" to talk about sin. Most people, even those who think they are pretty "sin free" will admit that they are broken in one way or another. And we'd be fools not to admit that the world is broken, that families are broken, that relationships all around us are fundamentally broken. And even the most ardent agnostic would likely say "If there is a God, that God certainly didn't intend for the world to be like THIS!". (and that may be why they are an agnostic in the first place.) And in fact, the world is not the way that God intended it to be. He created the world for wholeness, he created us for wholeness, and we (from the very beginning--hence "original") break the world, break one another, break ourselves. And even our attempts to make it better, to make ourselves better, are broken too and often end up doing more harm than good.
It's not just that individuals break rules (or don't) but you and I and our relationships and this crazy system we've come up with to live together and the whole of human existence is, deep down, fundamentally broken and not what God had in mind for us. And we don't have a clue how to put it back together again. And still, we're not willing to let God be God for us. We don't trust God to handle it, we want to do it ourselves. Adam, you dummy, why did you let the good life slip through your fingers just so you could do whatever the hell you wanted to do? And, for that matter, why do I?
The Good News is that, despite what many of us have been taught, God is actually not so worried about our rule breaking "sins" but in our inward curving that leads to them. God is interested in putting the world back together, in putting us back together, in making us whole. God calls us to curve upward and outward. But God doesn't force us, God invites us. Invites us to embrace this Kingdom of wholeness that most reject--to be people of Grace, people of trust, followers of Jesus. Jesus' will was the same as God's will--for himself, for others, for the whole world. It goes back to Luther's concept of "free will" which is not doing whatever you want to do (which is how most of us define it) but having what you want for yourself and the world be the same as what God wants for you and the world. To be truly free is to put ultimate trust in God, not in ourselves.
To paraphrase the Order for Confession from the Lutheran Book of Worship: We are stuck in our brokenness and cannot make ourselves whole. We have made God's world more broken, when God calls us to seek wholeness. But when we face our brokenness, God, who loves wholeness above all else, loves us in the midst of our brokenness too, and loves us all into wholeness.