Exposure in Public Spaces
Many people consider the genitals to be inappropriate to reveal in public. In most locations on earth, exposing one's genitals is considered a crime. The social expectation is that someone who does expose their genitals is a "pervert" (another word for degenerate) and they are seen as a sex criminal similar to a rapist. This directly contradicts the true nature of the genitals and of the human experience itself.
Genitals are not entirely sexual. Obviously they play an important part in sexuality but a foot fetishist involves feet in their sexuality and yet we don't consider feet to be sexual. The truth is that no body part is entirely sexual in nature or entirely innocent either. Any body part can be seen as sexually stimulating and any body part can also be seen as casual and normal by another person. The vagina and penis, are full of nerve endings, but hands and lips are also full of nerve endings. Penises contain the urethra which allows the evacuation of liquid waste from the body and vaginas are the exit point of period blood. Although many people find these evacuations sexually arousing, most people find them to simply be casual experiences of being a human and do not consider these functions sexual at all. Exposing the genitals is not always an act of sexual stimulation, but sometimes simply an act of relieving oneself of the burdens of life.
Even if one does expose their genitals for some sexual thrill, the erect penis and the lubricated vagina are natural experiences that people go through daily. I can not speak for the predictability or unpredictability of natural vaginal lubrication but as someone with a penis, I know that erections are not always at-will or even during times of sexual arousal. Sometimes an erection just happens spontaneously without any stimulation at all, physical or mental. On the other hand, sometimes I am very sexually aroused and can have a hard time getting an erection. It is entirely natural to see an erect penis and there's nothing wrong with it.
Even if there's some secret pleasure gained by the person exposing themself, people already do bdsm in public all the time and wear sexual devices when in clear view of others. There's nothing bad or wrong about this. As long as they don't involve others in their sexual actions, then what could there possibly be to oppose?