"Wherever we direct attention, we expend creative energy." -Samael Aun Weor
Therefore, if you perform the sexual act with a lot of lust, even if you do not reach the physical orgasm, that energy is directed towards the fortification of desire.
Transmutation occurs when we control lust (to whatever degree we are capable) in order transform our energies by directing them through conscious attention.
While the beginning is very difficult, since we have a lot of ego, the more we acculturate our mind, heart, and body to tantra through activating our consciousness, restraining, comprehending, and annihilating desire, we gain purity more and more.
For thirty years I sought God. But when I looked carefully I found that in reality God was the seeker and I the sought. -Bayazid al-Bastami