Unitarian Universalism is neutral about sexual behavior being neither for it or against it. UU respects the person's right to conscience and the free and responsible search for truth and meaning. UU also insists on the inherent worth and dignity of every person. Having said that, UU leaves the topic of sex alone.
The main purpose of sex, of course, is procreative to assure the continuation of our species, homo sapiens. Mother Nature has also made sex enjoyable for the most part so that humans will engage in it. On the ego plane, humans have wrapped sexual behavior into a belief about special relationships which becomes called romantic love. This emphasis on special relationships when it comes to sex contributes to huge amounts of drama as evidenced in our songs, our movies, our TV shows, and literature.
When it comes to sexual attraction and behavior, the emphasis is on the body with less attention to the soul. Pornography carries this dynamic to the extreme wherein lust is stimulated by images and interactions which have nothing to do with the spirits of the people engaged in the activity. This emphasis is in violation of UUs first principle and the fourth and so may diminish the deeper awareness of the possible meaning and purpose of sexual behavior which may be to transcend the physical and enhance deeper spiritual communication. Psychologists have found that sexual satisfaction is not enhanced by mechanical actions but by the quality of friendship of the people involved.
And so we come back again to the question behind your question which is "What is the purpose of sexual behavior?" The answer is procreative and recreative and it is in the procreative aspect that unconsciously we experience the most guilt because we believe that we have usurped the creative power of God. God is the creative energy of the universe who uses us to extend God's creation and we should realize that this creative power is not ours, but comes from the Godhead working through us. Do we understand ourselves to be the extension of God's creative energy in the world when we engage in sex or do we think we have stolen this power to ourselves alone? It is this unconscious guilt that we have stolen the fire of creation from God that leads to the myriad rules that religions make about sex without being fully aware of what they are doing.
Unitarian Universalism is not a guilt inducing religion. It does not teach the belief about Original sin but of Original blessing. UU does not believe in sin, guilt, and fear as other religions do and so it is neutral when it comes to sexual behavior. Unitarian Universalism is one religion which is not into the guilting business. If UUs teach anything about sexual behavior, it is that sex should always be loving, and respectfully engaged in with an awareness of our contributing with the Godhead to the interdependent web of all existence.