You get boner because it is a natural for example a man saw a beautiful girl and started thinking about sex then his penis would get a boner
No - if a girl has a penis then she is physically male and thus doesn't have a uterus, as such she does not menstruate.
A transgender girl would already have a penis.
You may be confused. Girls do not have a penis.
It can be virtually nonexistent as a girl can be satisfied with out the use of a penis.
There is no girl who saw a mermaid. They are not real and do not exist.
The size of the penis has nothing to do with your ability to get someone pregnant. Its your sperm. So yes you can get a girl pregnant with a 4.5 in penis.
Let me tell you, If she's got a penis of any size she is not a girl.