The Zohar reveals that at midnight the Holy One Blessed be He and the Shechina in Malchut come together for unification. This unification is a secret of the upper spiritual levels. If couples make their unification at that time with proper meditation, they would draw holy spirit from the holy unification above.
Those who don’t consider the time and meditation can’t receive from the higher Light and their unification is unholy and without Light. They could bring flawed children.
Most of the unifications between couples are not for having children but the unification itself brings new energy that was never in existence. It should be holy because that Light would manifest in a couple’s lives.
The pleasures of the unification are on the physical, mental and spiritual levels. It’s like no other pleasure and even when the physical abilities or senses weakened or gone, the desire to compensate on the mental and spiritual levels grow.
When unifying to produce a child the focus of the couple should be on the mental and spiritual because that level affects the ability to draw higher soul to the world.
Husband and wife should make great efforts to have unifications at proper time with purity and meditation.