If you are a person who lives in a fantasy land and believes that your BF/GF is going to be your husband/wife in the future or that they will be with you till the end, then better wait until your marriage. Cause if you have sex with him/her with the belief that he/she is going to marry you, if he/she dumps you, you will be regretting your decision. Sorry to say this but marriages are not made in heaven. So you have to leave heaven and get down to earth. On earth, everything is unpredictable that applies to literally everything including your perfect relationship.
But if you are a practical person who acknowledges that most relationships are based on need and that when the other person's need is fulfilled he/she will leave you, just go for it. You won't regret it, you will find another relationship if your current relationship fails, you will get in someone else's pants eventually, no big deal, you don't have to make a fuss about it.
So why it became an issue in the first place?
Ancients didn't have effective contraceptive methods or condoms and pregnancy was expensive. Expensive in the sense that it was a time and energy consuming process and after pregnancy, you have to take care of the child too. If women were allowed to have sex with people before marriage in those times, it would have been disastrous to the population control. And humans by nature don't really like to rise other people's children (am not talking about all parents). So if you had a kid before marriage in those times, the kid was most likely to be disowned by the mother after marriage. This practice can be seen in villages even today. So to avoid that ancients restricted women from having sex before marriage.
Now back to the present, who is stopping you anyway? If it brings you happiness and is with the person you like or fancy, why not?? As long as you are aware of what you are doing and doing it properly without hurting anyone in the process, with consent and caution, just do anything you want.
Or if you fancy some BDSM shit, even if it hurts just go for it.
Fifty shades of Grey da
Love me like you do da