Here are some interesting quotes from the 14th Dalai Lama regarding sex as taught and understood in the context of Tantric Buddhism.
“Being able to have sexual contact without releasing semen is something needed when you practice the advanced stages of the complete stage." - The 14th Dalai Lama (Berzin Archives)
"For Buddhists, sexual intercourse can be used in the spiritual path because it causes a strong focusing on consciousness if the practitioner has firm compassion and wisdom. Its purpose is to manifest and prolong deeper levels of mind (described earlier with respect to the process of dying), in order to put their power to use in strengthening the realization of the emptiness. Otherwise, mere intercourse has nothing to do with spiritual cultivation. When a person has achieved a high level of practice in motivation and wisdom, then even the joining of the two sex organs or so-called intercourse, does not detract from the maintenance of that person’s pure behavior..."
"Through special techniques of concentration during sex, competent practitioners can prolong very deep, subtle, and powerful states and put them to use to realize emptiness. However, if you engage in sexual intercourse within an ordinary mental context, there is no benefit." - How to Practice, Way to a Meaningful Life, His Holiness the Dalai Lama, Translated by Jeffrey Hopkins
"Actually, [..] the sexual organ is utilized, but the energy movement which is taking place is, in the end, fully controlled. The energy should never be let out. This energy must be controlled and eventually returned to other parts of the body. And here we can see there is a kind of special connection with celibacy." - Quoted from "The Good Heart," H.H. the Dalai Lama
"There's a great difference between the movement of the regenerative fluids for two individuals engaged in ordinary sexual intercourse as opposed to a highly realized male yogi and female yogini who are engaged in sexual intercourse...
"In principle, the general difference between the two types of sexual act is the control of the flow of regenerative fluids. Tantric practitioners must have control over the flow of the fluids, and those who are highly experienced can even reverse the direction of the flow, even when it has reached the tip of the genitals. Less experienced practitioners have to reverse the direction of the flow from a higher point. If the fluids descend too far down, they are more difficult to control." - Sleeping, Dreaming, and Dying, by The Dalai Lama (1997, Wisdom Publications)
"Although I am using this ordinary term, sexual climax, it does not imply the ordinary sexual act. The reference here is to the experience of entering into union with a consort of the opposite sex, by means of which the elements at the crown are melted, and through the power of Meditation the process is also reversed. A prerequisite of such a practice is that you should be able to protect yourself from the fault of seminal emission. According to the explanation of the Kalachakra Tantra in particular, such emission is said to be very damaging to your practice. Therefore, because you should not experience emission even in dreams, the tantras describe different techniques for overcoming this fault." - The 14th Dalai Lama
"One night a number of dakinis (female deities) were gathered in Birwapa's room at the monastery. Other monks heard female voices through the walls and the next day, at a gathering in the big hall, the head disciplinarian expelled Birwapa from the monastery. Birwapa left wilingly, recognizing that the time had come for him to practice "union" and to develop the Great Bliss which penetrates emptiness through reliance on an external consort..." - The Dalai Lama's Secret Temple, by Ian Baker, pg. 167 (Thames & Hudson, 2000)