hump a cactus

[aaronsun666]

The Big Bang Theory Season 03, Episode 08

Scene: The camp site.
Leonard: I wish Penny didn’t have to work, she loves camping.
Raj: Yeah, that would have been great. You and Penny having sex in the tent while I sit out here and watch Howard hump a cactus.

Is there any story or something behind 'hump a cactus'? If not, then Raj's words would sound very abrupt/strange to me.

 

[Copyright]

To hump is to have sex with. To hump a cactus is not only painful, but a sign of sexual desperation ... besides being funny.

[Someone has to say it: You have to be pretty (t)horny to hump a cactus.]

 

[aaronsun666]

Thank you, Copyright.