a bit late to this thread but i will try and be helpful
A simplistic description of sin is selfishness - Any act where you put your self infront of God is sinful. So for example, if you gave money to charity to make yourself look good this would be sinful. If you gave money to charity quitely and in 'reverence to God' this would not (matt 6:1-4) might be the most relevant verse here.
Part of Matt 27 goes something like this; Love the Lord your God with all of your heart, and Soul, and Mind.
the basic point of the new testiment (and by some interpretations the old) is that No one does this. As Paul puts it in Romans 3:23 All have sinned and fallen short of the glory of God.
So the rough argument is that we need Jesus to save us from our sin.
Whilst laws and specific sins are often mentioned in the Bible i dont think they are ment to be definitive or all inclusive. IN Romans 1-3 Paul lays down a reasonably logical argument essentially saying that the Jews could not keep the old testiment law, moralists (both secular and religious) cannot keep their own principles or be saved by them, and that essentially we are all screw without divine help. You may not agree with this (fair enough) but unless a Christian does not recognise any authority from this book, they cannot believe that being a Christian is about following rules and going to heaven for passing the test.
Interestingly Paul makes the main (possibly ownly, he might mention it in another of his letters) new testiment reference to homosexuality. He lists it in a non comprehensive list of sins, for some reason. The general arguement you will get of more evangelical, as apposed to liberal Christians on Homosexuality is that it runs counter to the blue print laid down by God for Christian relationships - Adam and eve / Man and Woman, plus some refs from the OT, and Paul in the NT.
People often use terms such as taking the bible 'literally'. Interpretation of the bible is a little more complex that this. The bible wasnt written to 21st century westerners. It has a cultural and historical context. As an example in 1 Corinthians 11, Paul suggests that Women should cover their heads whilst worshipping God. Does this means all modern churches that dont do this are in the wrong? Is Paul a sexist bigot, possibly. An alternative view is that this was a cultural norm in greek/jewish society 2000 years ago. If you ( im guessing abit here because ive never been to one) a Koptic Christian Church, or a Church in a dominantly Islamic society there would be nothing paricularly controversial about women covering their heads (rightly or wrongly). Perhaps this passage was written to such a social context. I guess from this you could argue this passage has little contemporary relevance. Could you then suggest that the whole bible has no contemporary relevance? I dont think so. The method may have changed but purpose remains the same, pray to God with respect. When it comes into interpretation you have to try and work out what the original writer intended to convey to the original reader, and then see how this is relected today. Some parts of the bible need little interpretation because people and their problems dont change much, some are very culturally defined. Is homosexuality in the bible a cultural stigma of the day, or something that is still to be considered sinful today? I dont know, but i er on the side of caution, not because i hate homosexuals, I have shared a house with two active guys, and never judged them for it, i have no right too (Matt 7 1-6). For Christians there is a difference between how we view people out and inside the church (Cor 5:12). We dont really have any role in judging those outside the church, therefore, i support secular homosexual marriage, if the state however made it a legal necessity that religious groups (christian or other) should carry out same sex marrages with out question, this i would disagree with.
i havent written this huge post inorder to convert, or warp your minds, im trying to give you a persepective your free to disagree with. Its worth saying many Christians will have a differing perspective to me, and i can go into the reasons for that if anyone is interested.