This is a complicated issue, which involves a lot of theology. And it’s made more complicated because people used to “just know” what was right and wrong about this.
I mean, this is like explaining to a kid why it would be wrong to feed your little sister manure sprinkled with cocoa and tell her that it’s brownies. It might seem perfectly reasonable to a kid, who wasn’t super-clear on the concept of brownies. Brown, soft, chocolate – no problem, right? Such a thing wouldn’t even occur to an adult, because his ideas about food would be fully formed; so it would be hard to come up with an explanation that would cover all the bases. You’d be better off getting another kid to explain it.
The important thing is this: Jesus did not die and rise to save only our souls. He came to save our bodies also. Body and soul, we are transformed into the Body of Christ when we are baptized. The life of Christ in our bodies and souls is fed by the Eucharist. When we die, our souls go to Heaven/Purgatory or Hell, while our bodies rest. But when we are resurrected, our bodies and souls will be joined back together forever – with our bodies remade by Christ for eternal joy, or by our sins for eternal separation from God.
The reason saints’ relics can heal and be associated with other miracles is that the saints’ bodies are more fully transformed into Christ’s Body. In fact, many many Fathers and theologians say that you cannot count relics as parts of dead bodies; they are live bodies, living and acting in the world. And that’s why there used to be a Feast of the Holy Relics on the calendar in many countries. (November 5, in case you wondered.)
Most of us are not that far along in the process of becoming Christlike when we die, so our bodies are sacred but not as sacred as relics. Any human remains deserve respect, as part of natural respect for humans as being made in God’s image and likeness. But the remains of baptized Christians (or catechumens, or so on) are only to be buried in holy ground around a church, or kept inside a church or chapel or consecrated cemetery (which also are holy ground, of course).
If you have a stately manor with a consecrated chapel, it’s okay to keep Uncle Bob on a shelf in your consecrated chapel, because he is one of the Christian dead and that is holy ground. But you can’t keep Uncle Bob on a shelf in your unconsecrated bedroom or around your neck, because that isn’t holy ground.
If you have a saint’s relic, you ought to treat it with the same respect you would show to the living saint, because that saint is partly living with you. But St. Bob isn’t really dead, so you don’t have to keep his body, or any parts of it, on holy ground. (He is the holy ground, in a sense.)
If you truly and actually don’t know any better, then it’s not a sin.
Given that (till recently) cremation was only done in the West out of disrespect for the Resurrection and a hope of never coming back, it is hard to understand why the Church giving grudging permission for cremation has turned so quickly into such strangeness. I guess the moral of the story is that if nobody teaches what’s right, people really do act like sheep without a shepherd.