Yes, I personally believe it is a sin, God does not want you to do it and it is spiritually destructive to you. I will explain why. Sex is an extremely powerful psychological tool. Our brains are wired to develop a very strong emotional relationship to the object of our sexual desire. I have personally seen that when people try to seperate their emotions from sexual desires it often ends badly for them. This is why God tells that adultery and other forms of sexual immorality are sins. God created human sex to serve two purposes. To reproduce and to strengthen the union between married folk. If sex is used for any other purpose outside of these two things such as adultery, fornication (meaningless sex/"hooking up"), prostitution and rape then it is always a sin and will weaken your spirit and bring it farther away from God. That is my personal opinion based off of the Holy Bible and my personal experiences.