Premarital sex is not a sin in and of itself. This misconception is causing a lot of harm to peoples' lives and needlessly chasing people away from God. There are times that it can be sin, and some need more moderation with it in light of the modern problems with hookup culture. At other times it can be a blessing to others, a wonderful gift to enjoy and share, and foster closeness. But for now, let's look at the main argument in this debate.
-"Fornication Argument": Much of this debate centers around 1 word: "fornication," often translated as "sexual immorality" from the Greek word porneia. Those who insist that premarital sex is always a sin base such on this word. The problem is they use circular reasoning. When pressed on the matter, they say, "premarital sex is sexual immorality because sexual immorality is premarital sex." That's circular. Others think that the word "porneia" itself means mere "premarital sex." Almost no published Greek lexicon says such. Neither usage of this word in the New Testament, nor in the LXX (Greek Old Testament), nor in extrabiblical usage supports such a definition of it. For premarital sex to be a sin, there has to be a vice attached to it such as: rape, cheating, violation of dowry code, etc. or anything that is unloving towards another.
It usually referred to a low form of prostitution that often involved trafficking women for sex slavery and at times idolatry. It would be wrong for a Christian to take part in this abuse. Sometimes it more broadly referred to "whoring oneself out to any sort of wicked lust," idolatry for example is a common usage. Whether idolatry, adultery, or sex trafficking, fornication could encompass these things and Christian's weren't to take part in this.
-"1 Corinthians 7:2": When one mentions the above, this verse is brought up at times. It reads, "But, because of sexual immoralities, let each man have his own wife, and let each woman have her own husband." It's assumed that this implies premarital sex is a sin since marriage is portrayed as mitigating sin here. Note however that it is in the plural ("immoralities"). It's not just one type of sin that is the issue but a variety of different types of sin. But what types of immorality? While it could encompass any sin that the word porneia indicates, the context points to a more specific answers.
Just a few verses earlier in the near context of 1 Corinthians 6 the word porneia is used there and refers to prostitution. Christians are warned to stay away from this form of abusive practice (which often involved sex trafficking and idolatry).
There is also the historic context. Most women were not free to have sex (unless consent by whoever was in charge of her care and dowry was situated). If a young man wanted to be with a young woman (or vice versa a young woman wanted to be with a man) this was not permitted unless the man in charge of her care (often her father) gave consent for such. To violate this was considered a form of porneia (immorality) that was a serious offense to the family. This is actually called moicheia (adultery) in Classical culture. If the woman was married, she would often be off limits too since again the male in charge of her care (her husband) did not always consent to another being with his wife.
Thus, an unmarried man would be tempted with different "immoralities" as few options would be available to him. He could be tempted to: 1. be with a woman without a father's consent (culturally insulting to the family and the woman and considered theft), 2. commit adultery with a married woman, 3. be with a woman trafficked into sex slavery and/ or a temple prostitute tied to idolatry. Few women were free to be with a man and those who were free legally were often not free financially but worked as hetairas (sort of like modern very high priced sugar babes, an often upscale companion who wasn't normally a slave but a free woman charging for sex). Thus, for many marriage would lessen this temptation of burning with no readily available way of having sex. Women too were often left tempted since often they weren't permitted to be with a man unless married (unless their father or husband consented or they were an independent woman which wasn't overly common). For more on this subject or to see citations and original sources that support the points in this post, see my book 40 Christian Myths about Sex.