Banana Sex is when a male and a female are about to have intercourse and the male doesn't want to 'stick his penis up in it' so he grabs a banana before heading in to 'do the nasty' and lays her on her back, both have their pants off, while he slides the banana in her. Then he has one of two options;
1] Continue till she orgasms. Then leaves it in her and runs.
2] Get up and run, after shoving it in her.
Banana sex is a mean gesture, usually done to embarrass women.
"Dude, i just had banana sex with that chick."