the obvious answer is,
they engaged in COITUS.
The Big Bang Theory Season 5, Episode 1
He asked her
during COITUS.
The Big Bang Theory Season 5, Episode 24
Technically,
that would be COITUS interruptus.
The Big Bang Theory Season 1, Episode 1
And she found atomic spectroscopy boring.
I wouldn't COITUS her with your genitals.
The Big Bang Theory Season 9, Episode 8
Can we please just stop saying "COITUS"?
The Big Bang Theory Season 1, Episode 1