[Yo, HC -- Biology is only one of numerous fields of science that have something to say about human sexuality. There you will find, and I think ultimately agree, that sex (sexual instinct, libido) is about much MUCH more than mere human reproduction.]

Psychology and sociology are both subjective fields. These fields are similar to science because they are made of theories, and philosophies. The difference between the science fields and the sociology fields is that sociology is entirely made up of theories. Psychology by the way is a mixture of science and theories.

Keep in mind that both of the fields you mention back up the fact that sex is for the purpose of reproduction. Psychology explains how chemicals and hormones in the body and brain gets the human male and female bodies ready for intercourse and possible child birth. Sociology basically explains the same theory that I have mention before, which is that human beings around the world use sex for different reasons despite its role and purpose in nature.