One of tricks of the enemy through the centuries is to try to blur the line between what is clearly sin. We can see this pattern at work throughout a reading of the Old Testament as the Israelites attempt to establish themselves as a nation under God.
Even today, our generation will tell us things are okay that are in exact contradiction to the Word of God. If we haven't been in the Word of God for ourselves, we may get sucked in.
Even our own hearts decieve us. The Apostle Paul told Titus that even our individual consciences can be seared, defiled and rendered useless from the effect of our sins (Titus 1:15). We become worn out and useless in our spiritual sense, unable to discern what is of God and what is in direct contradiction to God's character.
When things get blurry, we must go again to the Scripture. We must rediscover who God is and what he says to us. In his mercy he gave us his Word, but it's up to us to learn what he wants for our lives.