There can be a lot of factors that express sexuality. The origin would be its genes. The genetic make-up of the person what trait has passed on to an individual from his parents. Another factor is the environment. A person's sexuality can be influenced by social groups, home environment and from the media that he/she looks at.
        
             
        
        
        
The branch of biology dealing with interactions along organisms and between organisms and their environment is called Ecology, I believe.
        
             
        
        
        
When the food hits our tongues it triggered saliva (chemical digestion) then our mouths chewing is mechanical. So therefor your answer is The Mouth. Hope this helps <3