If you don't believe that premarital sex is adultery then you are much like the Pharisees. They were so consumed with being good on the outside that they totally neglected their heart's condition.
So people now might say "I just do it for pleasure." or whatever Jesus' words still are true. "But I say unto you, That whosoever looketh on a woman to lust after her hath committed adultery with her already in his heart." Matthew 5:28. So if just looking at a woman lustfully is adultery than sex, in any form outside of marriage, is this and much, much more.
It is also pretty clear from the OT that sex was intended for husband and wife alone. Notice the the wording in
Genesis 2:24 "Therefore shall a man leave his father and his mother, and shall cleave unto his wife: and they shall be one flesh." It says "they" shall become one flesh, referring to man and "his wife". So by using the word "wife" it cannot mean anything except marriage.
And if you have any doubt as to what "one flesh" means then 1 Corinthians 6:16 should clarify. "What? know ye not that he which is joined to an harlot is one body? for two, saith he, shall be one flesh."
So sex is not some neutral act that can be just done for pleasure but it is a unitig of the flesh.
Sorry for all you who hate religious descussions.