There have been times -- not as many as you might imagine -- that the "is a sin/isn't a sin" binary judgment is the best way I've found to answer a question. More often than not, though, life is so complex and the Christian ethical witness is so diverse that -- in so many of those moments -- it's a struggle to give a definitive answer. In those times -- which, again, is often -- I find the Wisdom literature of the Old Testament to be a better framing than the Protestant worry about escaping the wrath of an angry God.
Specifically, some things might not be sinful, but they can be decidedly stupid. Some things don't make you a bad person, but they can mark you as foolish. And some things aren't as immoral as they are immature.
Here are some advantages I've discovered using a wise/foolish frame over a not-sin/sin frame.
1. You avoid moralizing. Sometimes you don't want to be preachy, paternalistic and schoolmarmish.
2. You force them to take responsibility. We're never disinterested interlocutors. And very often we're acting in "bad faith", to use Satre's term. That is, we're running from our freedom and responsibilities. We're shopping for an endorsement from an authority figure when we need to start taking responsibility for the hard work of moral discernment and the resultant consequences of our behavior.
3. You avoid binaries. These moral decisions often come in shades of grey. Think about, as an example, the classic youth group obsession about where "the line" is in regards to sexual activity. Sin versus not-sin is hard to impose on these underlying continua. Wise/foolish is better situated to handle the range of choices and situational complexities.
4. The frame is more holistic. The focus on sin is often a narrow concern over God's judgment. The focus on wisdom forces you to consider a larger and longer view, how this particular behavior or path will affect me now and over the long haul. It forces you to think about things like character and happiness. Who do I want to be? Looking back, will I feel proud of these choices? Will this behavior get me to where I want to go?
Interestingly, while it can seem that the wisdom frame is downplaying the notion of sin it is, upon consideration, making the notion of sin more robust. Think about it. Why does God prohibit certain activities? Because God is a Puritan? Or are God's commands forms of care? Spend any time at all with the Torah and the Psalms and you quickly become aware that God isn't a schoolmarm, prude or finger-waving do-gooder. The commands of God are wisdom, patterns of life that promote psychological and social well-being.
You want fold to avoid sin and be good?
My recommendation: Spend less time on the sin lists. (This is, ultimately, why the medieval penitential tradition needed the virtues of Aquinas.)
Teach them wisdom.