Many years ago, as a young man, I faced this same question, and I'll never forget the advice a godly old preacher gave me: "When you're wondering if it's OK to do something that's questionable - don't do it!" Over the years, I've found this to be good advice, and I've always tried to follow it. I often recall the Bible's words: "Abstain from all appearance of evil" (1 Thessalonians 5:22, KJV).
If God has told us that something is right or wrong, we need to obey His Word without compromise. The Ten Commandments, for example, are still in effect, and if we ignore them we will pay a terrible price. (You can read them in Exodus 20:1-17.) Jesus said, "If you love me, you will obey what I command" (John 14:15).
You're right, however; the Bible doesn't give us an exact rule for every situation we might face. But the Bible does give us the moral and spiritual principles we need to guide us. For example, the Bible tells us to "Make every effort to live in peace with all men and to be holy" (Hebrews 12:14). How much better our lives - and our world - would be if we followed those principles!
No matter what others do, make it your goal to live for Christ and follow Him. Do you simply blend in with everyone else? Or do others see Christ's love and peace and purity in your life? Ask God to keep you pure - and to make you a witness to others.