While pleasing a girl, to stick your thumb out like a hitchhiker and plug it up her vagina. Very simple yet effective.

When you borrow your friend's shorts/pants after he had just jerked-off in them. You are now harboring a "hitchhiker"! You then go home to your wife/gf and do the dirty, 9 months later you have a kid fathered by none other than... Your best friend!

When you are with a female and you are giving her oral and you realize that she would rather rub one off herself. You proceed to allow her to rub one off while you extend your thumb and stick it in the vagina while she is rubbing so that you can feel all the goodness during her orgasm.

To the definition above. NO! HITCHHIKING IS NOT A GAY SEX ACT YOU RETARD!
Anyway, hitchhiking is the method of transport/lifestyle in which a person or group of people stand by a major motorway and wait for a free ride. It is risky but the likelihood of being raped and chopped into tiny bits is very small and overplayed by the fear-mongering media. In my opinion.

A hitchhiker is an individual who travels the universe without the use of a ship of their own. Popular hitchhikers are Arthur Dent and Ford Prefect. Contrary to popular belief, Zaphod Beeblebrox is not technically a hitchhiker because he owns a ship which he has stolen (see: heart of gold)

Also, it is a common misconception that a hitchhiker is someone who knows where his towel is. This fact is only true to froody (cool and together) hitchhikers, giving rise to the popular expression "that frood really knows where his towel is". Arthur Dent and Tricia McMillan are two examples of "froody" hitchhikers

to get free rides by standing next to the road and signaling drivers

One who knows where his or her towel is.