The Church is very clear. Full, complete, genital sexual activity is reserved for those in the sacramental covenant of marriage.
Let me gently challenge the “where should the line be drawn” type questions. Love is not a reality that is measured and molded by rigid rules. The reality of love, God’s very self, is the transformation of human persons into beings who can live forever with God. That transformation begins when we are conceived and is marked by our Baptism. Any and everything we choose to do should be in tune with that transformation of our hearts and minds and souls.
The challenge for those who are not married is to find appropriate and just ways of expressing what their relationship means. The “100 ways of doing it without having sex” pamphlets show the real charge comes from connecting, being intimate, “at one” with another person. We are made for love by love to love unto love. There are many ways besides getting naked that better help us connect.
Most people find conversation the most potent mode of really connecting with another. Too many jump into bed because they have nothing more to talk about. When dating, strive to really connect. Keep the conversation going and deepening. Reserving sex to express when that connection has deepened into the desire and commitment to form a lifetime union makes sense on many levels. Waiting until one is married really does express the sacredness and specialness of giving totally one to another, body and soul.
Our culture too often mocks such views of sex. The problem is that all those who are having sex whenever and as often as they can, often reveal that there is still a part of them that is empty and yearns for something more. The solution is not to try and fill that void with more addictive or mindless sex. The solution is to sit with the emptiness and yearning, because when the right person comes along and fulfills that yearning, so much more than sex is in the relationship. The two become one and find God and joy and justice and peace and happiness in their love.