Another question you could ask is murder a sin? And if you decided to do it, knowing it's a sin...do you think God would forgive you?
If you confessed it and repented of it, yes, He says He would forgive you but you'd still have to deal with the consequences of knowing that you took a life.
Think about this, there's a reason why God says not to do certain things. And that is because they don't bring forth life. They bring hurt, and pain, and destruction and even death. It's not about if we can "get away with stuff" - just do it and then apologize and repent and everything will be o.k. There's consequences to what we do. When you sow weed seed, weeds will grow in your garden. And weeds drain the good stuff out of the ground. Do you really want to sow weeds in your heart that will drain the life out of your soul? Do you want to deal with guilt? Why take that which is so precious and play around with it so carelessly?
Sex is a beautiful thing that is meant to bring people closer together in marriage. To use it casually outside of marriage because it feels good is foolish, sinful, and being short-sighted.
Have you ever heard anyone say, "I'm really glad I slept around before I married my spouse!" ? Or "I'm glad we didn't wait until our honeymoon." ? I haven't and there's a reason. There's a bonding that takes place, both physically and spiritually when people have sex. It's a healthy and good bonding but it's sacred and that's why God designed it and requires that it be done in the arena of marriage. (And He wants marriage folks to do it A LOT and enjoy it A LOT!!)
If you opt out of His plan of sex in marriage, then you'll end up with not only a lot of guilt and regrets but part of you will be torn apart after that person leaves because part of you bonded with them. That's the way we're made and that's also why God hates divorce. He doesn't want our hearts broken up in slivers and strung out.
You may also want to consider what wounding you be inflicting on the guy's heart, (not to mention your own), by sleeping with him outside of marriage.
Sex is a serious business and it's a Holy thing that far too many people have taken way too lightly -like taking a rare jewel and treating it like cheap 25 cent toy from a vending machine.