So the Bible clearly tells us not to have sex if we aren't married.
HEBREWS 13:4 "Let marriage be held in honor among all, and let the marriage bed be undefiled, for God will judge the sexually immoral and adulterous"
After learning that & reading that verse I decided that it was definitely something that I wanted to follow. We are human and humans aren't perfect but I knew that God didn't want me to do it before marriage but I wish I could have understood WHY sooner. I found this verse and it helped me understand the why behind the verse above.
MARK 10:8 "...and then the two will become one flesh. So they are no longer two, but one flesh."
After I read this my eyes were opened to the depth and meaning behind sex. It is so much more than something to just mess around with or do with whoever we want. Even if you are in a serious relationship with someone or even engaged to them you never know what is going to happen, this world is super unpredictable and unfair. Creating that deep bond with multiple people does have a negative effect on you even if you don't see it now. The verse says that the two become one flesh, imagine becoming one flesh with multiple people...
Besides hurting yourself emotionally their are the obvious consequences of getting pregnant or getting a disease but you have probably heard people warn you of those things. It blows my mind how this world has twisted the meaning of sex. There is so much pressure on everyone to give into it, and if you don't you are looked at as weird or as a "goody goody" as I was once called. I know that guys literally say "Let's just do it once, it's not that big of a deal". PLEASE don't ever fall for that line. IT IS A BIG BIG DEAL. Don't be persuaded by anyone.
Please recognize your value. You don't have to do it. If you feel like the dude will leave you if you don't, be sure to show him the way right out of your life. He is NOT worth it and you deserve a man that will not only respect your morals but hold your relationship to the highest standard and he will show you love in other ways. Sex is not something to be taken lightly.