Simply means Anal sex. It is derived from the fact that Spartan men were forced to bond using anal sex and anal sex is often used as a birth control in modern society.
"Dude, you gunna need a condom tonight?"
"Nah, I'm just gunna stick to good old Spartan birth control."
When one finds himself without a condom or has run out, it is the act of anal sex. As anyone who has watched a lot of Euro porn can tell you, anal is as common in Europe as the high-five. And, we do thank-you for that.
"Look at the ass on that babe; I'm hoping for a littleEuropean birth control tonight."