We have had a couple of questions along the lines of "is X a sin?".
Should these questions be on topic or should we disallow them?
I think that these are valid questions. Such as What is the biblical basis to claim that masturbation is a sin?
As Christians we're not only looking for the truth about scripture, but also how to live our lives. This site will be a great resource for me in that area of learning.
I think we should disallow them. The simple fact is that these questions only lead to debate and argument, and have little real value. Really, it's just simply the wrong question to ask. In order to define whether X is a sin or not, one must determine WHAT sin is, and then the question of whether X is sin or not answers itself. Typically if a person is asking this question it's because:
A. Someone told them that it's a sin to do X.
B. They feel guilty about doing X but like to do it.
Are there passages in the Bible which can shed light on these things and give us an idea of whether it's good to do or not do certain things, I feel certain that there is in every case. However, most of these types of questions are either a matter of personal conviction or a matter of legalism, and any answers will be swayed severely one way or the other by these viewpoints. Sin is far more about the "Why" than the "What". Adam and Eve ate the fruit in the Garden of Eden. The fruit wasn't the sin, their act of rebellion against God's command was.
I grew up in a church that said that for women to wear pants was a sin. So, using that as an example, I know women who are personally convicted about wearing pants, and those women wear skirts all the time. However, they do not condemn any women for wearing pants, and will never say that for a woman to wear pants is a sin. There's a possibility that they shouldn't wear pants because they would want to wear them too tight because they're proud of their physique and God knows that and is teaching them about pride. Maybe He has a completely different reason for it.
On the other side of that coin are the women who refuse to wear pants because "it's a sin". Their viewpoint is that new Christian women who wear pants simply "haven't seen the light" or "haven't matured spiritually to that point yet", and they make it their mission to "help that person out of their sinful state". So, not wearing pants becomes a point of condemnation for these women to feel more spiritually "adept". This causes them to base a portion of their salvation on works, and on things they are doing, rather than on the single redemptive act that Christ did for them.
There is only one answer ever to "Is X sin?" and that is: "Are you convicted by it?" If you are convicted by it, then you shouldn't do it, but you shouldn't condemn others if they don't feel about it the same way you do.
Now, there are guidelines in the Bible about every aspect of life, and if someone wanted to ask "What does the Bible say about X?" I would be far, far more accepting of that question. As was exampled by Jonathon Byrd's answer, some topics have merit. Whether or not such topics are sin or not is a matter of debate, and personal conviction. I'm sure the Bible has some things to say about it though.
This is going to be a tough one. My first idea (feel free to rag on me if you disagree) is this:
Already in writing that I can think of a bevy of exceptions, but it might be a pattern at least worth having around for any really context-less "is x sin" questions that come along.
I think they ought to be in there, you might as well ask "Where is Contracption in the bible", where is "Masturbation in the bible".
I'd rather the question be framed "Is X a sin?" or better "Is X grave matter?" because if the answers aren't covering all the bases, I can add a Catechism reference (with appropriate personal comments).