Adults do tend to undergo penis and testes contraction, however, in times of fear or extreme subordination. There is a remarkable similarity between the comfort postures chosen by human males and those baboons sitting out there on a bare anthill, or monkeys in tall shrubs. It is mildly verboten for a male to heist one leg up and place his foot on the chair though this is quite comfortable. In Body Language, Julius Fast analyzes leg-crossing and its importance in social signaling, but doesn't refer to its origin. Leg-crossing is a symbolic gesture of hiding the naked self.
Males tend to use the legs-apart postures more frequently than females.
I don't mean to imply that girls had no say in who wasn't a desirable boy to date, but they were surely influenced a great deal by the stature a boy had among other boys. One has only to look at how frequently the team captain dates the pretty cheerleader, the homecoming queen, or some analogous pairing. Also, a woman plays a definite role in the man's self-image. Some of the psychoses men with small genitals have must derive from their fear of disappointing a woman - not necessarily in her orgasmic satisfaction, but just in her getting a man with smaller male organs that might be incorrectly interpreted as less masculine. Often these are small men, men with small genitals, men conspicuously lacking in other organs of status, or men with some major social handicap.