About the origin:

link - make

link - love

Take make and love and blend the two etymologies thoroughly and you will have a whole bunch of aspects as to the meaning of make love through time.

(this one is pretty too: ( also from online Ety..)
make (n.)
"match, mate, companion" (now archaic or dial.), from O.E. gemaca, from P.Gmc. *gamakon-, related to gemęca "one of a pair, mate, consort," gemęcc "well-matched, suitable," macian "to make" (see make (v.)