The question was posed by one sex researcher: “Did you know that bedbug males penetrate the exoskeleton of females to fertilize them?” Another researcher responded that this question, and wrote that it brings up the age-old question: Which came first — the penis or the vagina? He jokingly added that the answer is pretty clear: it was the penis. This video of “penis fencing” in marine flatworms supposedly exemplifies a fact. There are …four penises flailing and not a vagina in sight: https://www.youtube.com/watch?v=5fx-YgcP8Gg
I recognize the joking nature of the above exchange, but it brings up at least three more important questions, which many sex researchers have not addressed.
- How did the sight of a vagina become important to sexual behavior?
- What animal model suggests the importance of visual input is greater than the importance of chemical input either in food choice or mate choice?
- Is penis fencing in worms an indicator of the relative salience of sensory input?
In the nematode, Caenorhabditis elegans, for example, recent results suggest that even the importance of food odor is not as important to behavior as is the organism’s already ‘hard-wired’ response to the odor. “This was a surprising result because most people thought that sensory information was perceived as neutral, with the brain deciding later from the context whether it was good or bad. Some scientists said that only worms behave this way, but the same result was later obtained in mice.” 
1. Given this extension from a worm to a rodent, and
2. the fact that the rodent’s reproductive sexual behavior seems to be driven by its ability to acquire food, and that
3. food acquisition determines its production of social odors (as appears to be consistent across all animal species),
4. the leap to the importance of visual input (e.g., the sight of the vagina) in men seems to be a huge leap that is unsupported by at least three steps that are required to link a sensory stimulus, like a chemical stimulus, to behavior.
But wait, I think that both sex researchers were joking, and so am I, albeit only somewhat. From the perspective of molecular biology, it is clear that ligand (e.g., penis) – receptor (e.g., vagina) interaction requires the co-evolution of both. However, it does not require the evolution of vision.
How then, are sex researchers going to explain their continued focus on the importance of visually perceived physical characteristics of the opposite sex (e.g, the vagina) with regard to behavioral development in humans? The overwhelming importance of chemical stimuli in this regard is a fact of life across every species that reproduces sexually.
 (another question: How does the brain of any mammal decide, in context, whether sensory input is good or bad?)
 The overwhelming influence of chemical stimuli does not seem to require that a specific receptor be identified, for any particular ligand (e.g., a pheromone). The observed behavioral response may be the only evidence that ligand-receptor binding has occurred. That doesn’t mean that if we don’t see the response, it doesn’t occur, or that we must see anything for it to occur.