As a good initial principle here, we should affirm that sex itself (and sexual activity in general) is not inherently negative or sinful.
On the contrary, in the proper context, it is a kind and good gift of God.
With respect to pre-marital, romantically oriented kissing, we're clearly talking about an area about which reasonable believers can (and do) disagree.
Let me lay out what I view to be applicable biblical principles and passages on this topic.
It is certainly true that no passage of Scripture says — in so many words, at least — "thou shalt not kiss before marriage." Having said that, I submit that there is a strong argument to be made from Scripture that there is sexual relationship outside of marriage.
I am obviously not saying that hugs and kisses of affection or greeting to relatives and the like are out of bounds. In some cultures, kisses of greeting — between members of the same sex or of the opposite sex — as well as hand-holding and other forms of physical expression during normal, non-romantic social intercourse, are more common. You might even be able to talk me into the notion that , "non-leaning-in" hugs of greeting, sympathy, etc.
between men and women who are not romantically involved are OK.