There are many gradations in-between these two extremes, and it is possible that what is sinful for one person will not be for another, depending on their thought processes.Furthermore, sustained kissing is considered foreplay as preparation for physical intercourse.
As Christians, our efforts should NOT be toward pursuing physical pleasure, but we should rather chase after godliness and all the other traits of a believer.
Second Timothy says, "So flee youthful passions and pursue righteousness, faith, love, and peace, along with those who call on the Lord from a pure heart." Even when you're married, sex should never be the #1 priority in your relationship.
It may be the first "stop" for the "train" that ends with penetration and orgasm at the final "destination," but it is still part of same general process.
It can easily be a sexually charged "warm-up" that prepares for consummation even when the couple ultimately avoids going that far.
God is faithful, and he will not let you be tempted beyond your ability, but with the temptation he will also provide the way of escape, that you may be able to endure it" (1 Corinthians ).