It is particularly difficult to make these assumptions when the topic is considered to be private, or rational reasoning is limited due to the emotion or fear of the topic. People should do good and avoid evil. Those things that people are naturally inclined to do are most likely good. Good ought to be pursued and evil avoided. Good ends are those towards which people are naturally inclined.
People are naturally inclined to sexual intercourse.
A natural purpose of sexual intercourse is reproduction. To act intentionally against the good of reproduction is evil and ought to be avoided. The purpose of reproduction is continuation of the species. Not everyone must have children to do this so it may not follow that reproduction ought to be pursued in all sexual acts. Many philosophers think not.