The urge is derived from internal electro-chemical signals, a product of the endocrine system.
Are you concentrating to much on the socio-conceptuality of the issue. Wherein, you feel the need to justify a certain aspect of human behaviour.
I would suggest, a behaviour that despite concerted modern efforts, is still deeply rooted in archaic taboos.
And let me just stress, I care not who does what with who, in the bedroom or on the kitchen table.
As long as they are all consenting adults.
I have a liberal view of sexuality.
And perhaps a more conservative view of human biological systems..