Sex as Yajna:

Yajna or sacrifice is derived from root word “Yaj”. It means Worship or offering an oblation.
Max Muller defines Yajna is an act by which we surrender something for the sake of gods”6.
Sex is worship. It is an act by which the couples surrender their ego, in order to gain pleasure, progeny, eventually even enlightment.

“Her lap is a sacrificial altar; her hairs, the sacrificial grass; her skin, the soma-press. The two labia of the vulva are the fire in the middle. Verily, indeed, as great as is the world of him who sacrifices with the Vâjapeya (“Strength-libation”) sacrifice, so great is the world of him who practises sexual intercourse”7 (Brhad-Âranyaka Upanisad)

These verses clearly shows that, sex was treated as a form of worship, an act to not only fulfill one”s desires and gain pleasures, but also as an act of sacredness.

Sex as Meditation:

In Vigyana Bhairava Tantra8, during a conversation between Shiva and Shakti,

Devi Asks:

O Shiva, what is your reality?
What is this wonder-filled universe?
What consttutes seed?
Who centers the universal wheel?
What is this life beyond form pervading forms?
How may we enter it fully,
above space and time,
names and descriptions?
Let my doubts be cleared!

So, Shiva explains her 112 methods of meditation to attain enlightment. He says-

At the start of sexual union
Keep attentive on the fire in the beginning,
And so continuing,
Avoid the embers in the end.
When in such embrace your senses are shaken as leaves,
Enter this shaking.
Even remembering union,
Without the embrace.

These verses clearly indicate how sexual act can be utilized for achieving enlightment. Enlightment is a state, when all egos vanish. As in a sexual act, the couple leaves behind their ego and unites with each other and achieves sexual ecstasy. This very thing can be utilized to achieve spiritual ecstasy.