Are you sure?
I think if you marry, your body belongs not only to God, but also to your spouse. So if you sin against your body, you sin against your (future) spouse also, as you treat your body not the way your spouse would want it. That goes for watching pornography or masturbation also.
Your body belongs to your spouse AFTER you marry. Not before.
When you marry you take the body that you chose to marry AS IT IS NOW. It may carry baggage with it, and its for you to decide if you want to marry or not.
There is nothing biblical to say that you can sin against someone you've never met and do not know, but even if it where possible then think about this. . .
If pre-marital sex sins against your future spouse, does not dating other people prior to you getting married also mean you have sinned and cheated on your future spouse??? Because when you marry EVERYTHING save for your eternal soul belongs to your spouse.
If dating another person before you are married is a sin, then every single time you go out on a date with a person you don't marry you are sinning.
Pre-marital sex isn't a sin against a future spouse, its a sin against God and his intentions for human sexuality.
The other major problem I have with this idea is that if we hold that pre-marital sex is a sin against the future spouse, then by logic a lot of people use, they would say "Well we are going to get married, so because we are planning on getting married, we can go ahead and have sex." Thats an out and out heresy that this line of thinking produces. And its been used as an excuse more times then I can count. And those relationships have also failed to marry more times then I can count too.
But if pre-marital sex is a sin against God, not some future spouse, then its impossible to have sex before you are married without sin. Because its not about the WHO, but about the WHEN. God intends for us to wait for the married relationship, not to wait for the person we think we are going to marry.
If I had had sex with my wife prior to marriage (which we didn't) the fact that we are married now would not have made that prior sex somehow OK. Because its God's intention that sexuality only be used in a married relationship.