I am not very much into tantric sex but just wanted to share something.
Many Guru's have said (and I agree) that its like changing the flow of a river upwards that is flowing down from a mountain.
You are making a dam down there to produce electricity.
Every man is 50% and so is every woman. A boy sees a girl who has a nice body and says "Wow! she is soooo hot!" and forgets the whole world and his whole awareness goes onto her.
Why does this happen?
Because the girl has got something that he does not... the other 50%. A curvy body... etc etc...
So does a girl when she sees a boy that has her missing 50% criteria. But still, girls are less interested because they have menstruation that releases the urge to find the other half momentarily automatically. Boys have to do it themselves... what miserable creatures... hehe.
The release of blood/semen is required continuously to update the genetic/dna for proper evloution. So that the new generation is always genius and ahead of us. Otherwise we would have remained monkeys till now.
Well, at the time of ejaculation we are close to God because with his divine energy we become creative to produce new life. But this new life again is 50% (either a girl or boy). Strange!
After ejaculation we get relief (or feel sad/guilty), you touched eternity for a moment and then it goes away. You also become good after that... for a while.
Tantra says until we have the inner intercourse (Shiva meets Shakti)... you will remain two, dual.
I guess if one reaches the stage where his/her witnessing becomes his/her normal conscious state (as Chinna said), and then takes the path of Tantric sex... this inner union will become a lot much easier then.
Yes, beyond the witnessing, no concern will arise about complexities like tantric sex and Siva-Sakti, as ways of relating. 'The ego-I is the illusion of relationship', as a book title once put it.