Is premarital sex a sin if you are engaged and end up marrying that person and are married to that person?

for the rest of your life?Is premarital sex a sin if you are engaged and end up marrying that person and are married to that person?
Yes because marriage is a covenant and sex is not permitted before the marriage or covenant is made.Is premarital sex a sin if you are engaged and end up marrying that person and are married to that person?
I don't think so, also I think once you have sex with someone, you are married to that person because you and that other person became one and are bound together, even if your not married with papers and all that. I think it's wrong if you have sex with more than one person, because the person you have sex with should be the one you truly love and want to stay with forever.
It's not a sin no matter what.