I want to have sex with this guy but I know it's wrong what do I do
You trust your instincts, in this case, God's Spirit leading you, NOT to do what your flesh wants you to do. We all face temptations, that's normal and expected. And we have to make those critical decisions of whether or not we will give in, each time. The strong stand by their convictions and the weak don't. Ask God to give you the boldness and strength to turn away. Sexual temptation is perhaps the strongest temptation you will ever encounter. And because of this, the bible warns us not to resist, but to get away from it completely (1st Corinthians 6:18). Don't think you're strong enough to just resist, but make a conscious effort to never put yourself into a possible situation that would allow this to happen. Focus your mind on good things, things that are pure, lovely, and admirable (Philippians 4:8). You can't allow your mind to continue to entertain this temptation, these feelings and thoughts and expect them to go away.