This is an all too common opinion voiced by too many people in the world; including unfortunately, many Christians. Invariably, it is used to justify some form of action or behavior, which is questionable. It is also used to dull or supress feelings of guilt when trying to convince oneself that if we are not hurting anyone else, what does it matter? So, what does the Bible say about it?
1 Corinthians 6:19-20 (NASB)
19 Or do you not know that your body is a temple of the Holy Spirit who is in you, whom you have from God, and that you are not your own? 20 For you have been bought with a price: therefore glorify God in your body.
God’s Word is clear: we are His. If the contemplated action or behavior does not bring honor and glory to God, then we should not do it; even when we are alone in the privacy of our own homes.