I made a promise to God that I would not have sex before I'm married, but my friend has told me that the Bible says if you love that person and you are getting married or won't break up then its fine. They say my promise makes no sense because it doesn't say anywhere in the bible that you are not allowed, but I can remember reading that it says sex is a gift from God to married couples but I cant find it. I was wondering if you could tell me why the Bible is giving me mixed answers. Please help me try to understand wut God wants. Thanks.
The obvious scripture that comes to my mind is Hebrews 13:4, which says "Marriage is honourable in all, and the bed undefiled: but whoremongers and adulterers God will judge." This probably is speaking specifically of adultery after marriage, but I think it also applies here. The marriage bed should be undefiled. That is, when you go into marriage and have sex in marriage it should be based on a trust that both parties have kept themselves for the marriage partner, or in the case of a second marriage that the previously married partner had been pure in his/her marriage. The last part of the verse specifically talks about those who would sell themselves or others before marriage or who would violate the marriage vows. If you have sex before marriage, you are selling yourself for a moment of pleasure against a lifetime of guilt.
I address a number of other passages on the same subject in an answer to another question. Go here for that answer.
If the person is telling you that the Bible says it is OK if you are getting married or won't break up, you should have them show you exactly where it says that. Anytime somebody says it is in the Bible, ask them to show it to you. Besides which, short of actually getting married, how do you know you will never break up? There are numberless times that somebody has had sex knowing they won't break up, and then as soon as the guy has what he wants he dumps her for someone else. The only way to make sure you won't break up is marriage. Even if the marriage breaks up, God forbid, then at least the sex was acceptable in the sight of God and not a sin.
I am glad you are asking such important questions first. Save yourself for the right guy, and then save the sex for after marriage. Then you won't have any reason to feel guilty. May God bless you in your attempt to follow Him.