The only way we
can determine function is
by observation.
Yes, intercourse does
function to pair up zygotes.
But observation
also shows many
additional functions of
intercourse. Hormones
are regulated
by it. (Through them, our thinking.)
Social bonds get formed
(and broken) by it.
And, in fact, only some times
do zygotes pair up.
But all times we see
these other things occurring.
We cannot dismiss
these other functions
and say just reproduction
is the "purpose" here.
(Complicated things
often play multiple roles --
Language is a tool
for thinking, AND it
works for communication.
Sex is not simple.)