Is SIN a sin just because God says it is sin? What then shall we say? That the law is sin? By no means! Yet if it
  had not been for the law, I would not have known sin. For I would not
  have known what it is to covet if the law had not said, “You shall not
  covet.” (Romans 7:7, ESV)


In the Old Testament, working on Sabbath was a sin because God said so. Eating pork was a sin because God said so. In the New Testament, we are relieved from some laws but adultery is still a sin.

Suppose, after the Judgement Day, we all live in a New Earth, and God says, 


  "There will be no more monogamy, no more marriage. You are free to sleep with anyone
  you wish, either male or female.",


will it still be a sin to do this?

Is "sin"  a sin just because God says it is sin?

And when God says it is no longer a sin, will it still be a sin?
Let's clarify the definition of "sin". Sin is often defined as performing acts that are contrary to specifically noted prohibitions in scripture. In the Old Testament, prohibitions against eating unclean animals, specific instructions about performing certain rituals and ceremonies, etc.

I believe the Biblical definition of "sin" is not so much about actions (dos and don'ts), but about our relationship with God. Sin is really unbelief, and as James taught us, our faith or our unbelief in God is manifested by the things we do (see James 2). 

In John 16 Jesus speaks of how the Comforter, when He comes, would reprove of sin not because of a failure to keep ordinances (which the Pharisees excelled at), but because of unbelief.

John 16:8-9
And when he is come, he will reprove the world of sin, and of righteousness, and of judgment: 
Of sin, because they believe not on me; 

The Pharisees and religious men of Jesus' day lived spotless lives according to the law, but they were the greatest sinners of all, because they simply didn't believe His message. Outwardly they appeared holy and righteous, but inwardly they were "full of dead men's bones".

Matthew 23:27 - 
Woe unto you, scribes and Pharisees, hypocrites! for ye are like unto whited sepulchres, which indeed appear beautiful outward, but are within full of dead men's bones, and of all uncleanness.

Performing various actions is often considered "sin", but true sin is failing to take God at His Word, which becomes manifested by our actions. To answer your question pointedly, failing to take God at His Word will always be "sin".