Unlike what some psychologists state, the basic core of sexual behavior has deep evolutionary roots. Think of sign stimuli and courtship patterns in people that may be largely universal. The effect of touch for animals depends on hormone levels. Again, hormone level is like an establishing operation that sets the occasion for touch to elicit sexual behavior. If the hormone level is low or the context inappropriate, exactly the same kind of touch is actually aversive.
Sexual context varies from individual to individual and even changes over time within individuals.
External stimuli can affect hormone levels too. The example of the ringdove on p. 124 is a clearly written example. It is entirely possible that hormone levels in humans can be sensitive to environmental cues. The anonymous report of one man's beard growth spurts occurring only in the time frame preceding his trips back to civilization and female's menstrual cycles synchronizing show support for this theory. Variety and the Coolidge effect.