A hermaphrodite is defined as any organism that has both male and female reproductive organs and this in human beings, is very rare indeed.
What is found in humans is vague reproductive parts that make it difficult for those humans to be properly placed as either male or female. That hermaphrodites can have sex by themselves and even reproduce is not possible because either or both of their sex organs are non-functional.
Naturally, sex is achieved with an erection that takes the manhood away from the body and therefore cannot be bent towards ones own body for self-sex.