Marriage is a union of love. Pre-marital sex is never accepted at all in the states. The Bible condemns it, and the scripture. Anything apart from this is not part of the plan or is confusing.
In Hebrews 13, And we have talked about how marriage is an honourable thing and where adulterers or fornicators will be punished because they defy their bed. This shows that sexual intimacy is supposed to be acceptable only in marriage. Individuals who are always fond of sleeping with each other before marriage are not going according to the teaching of God, although they may have been in love but this is not still legal to have premarital sex.
When we realize this as Christians then we can adjust our ways because sex is a spiritual thing and it hinders or stagnates our spiritual life when we engage in premarital sex.
The bible in 1 Corinthians 6:18 also advises us to flee from fornication. The reason for this thing is wrong. Our body is the temple of God and must always keep it clean. If we are not in a legalized union of marriage then we should abstain from any sexual intimacy with any individual. God wants us to have respect for our body and not share it to everyone as believers.
In Psalm 109:105. The bible said "my words are a lamp in our feet and the light to my path". We have to follow the teaching of God if we want to lead a fruitful life and sex before marriage is not accepted in the faith.