We're all sinners. And it's been my experience that many, if not most, single Christians struggle with their sexual needs. Paul wrote:
I Corinthians 7:8-9
8 To the unmarried and the widows I say that it is good for them to remain single as I am. 9 But if they cannot exercise self-control, they should marry. For it is better to marry than to burn with passion.
Paul understood a single person's need for sexual intimacy. Paul's advice was that it is good for the unmarried and widows to remain single and celibate as he was. However, "if they cannot exercize self-control, they should marry."
I find it interesting that Paul doesn't say they are bad people. Paul doesn't condemn them to Hell. Paul doesn't disown them as Christians. Paul simply indicates that if one's sexual need is so great that they cannot control it, they should look to marry.
Now...is it "sin"? Yes. Anything short of God's perfect will is sin. However, this doesn't mean that God hates those who struggle with their need for physical intimacy. It only demonstrates the fallen nature of man and shows us why the grace of God is so necessary for our salvation.
If any reader is struggling with this here... First... know that God loves you and cares for your situation. Second, turn to God and confess. Be honest with how torn you are. Cry out for His forgiveness. Ask that God help you overcome this struggle through the Holy Spirit. And consider that if these powerful needs do not subside... you are indeed meant to marry. Tie the knot and make your relationship something that brings honor to Christ and I believe that you'll find yourself feeling blessed.
If you find that you know that your relationship isn't a "lifetime relationship" that could become a beautiful marriage... end it. Save yourself the pain and heartache. Then pray that God help you find the one you're meant to marry.