A cheap (probably free) and meaningless gift that is meant to be an expression of love for your girlfriend, given with the sole intention of persuading her to have sex.
A typical sex coupon would be food or drink presented as heart shapes: cake, buttered bread, froth on a Cappuccino.
Another sex coupon would be something like a heart shaped key ring you got free with some expensive stuff you bought for yourself.
When drinking beer out of a bottle, one might try to peel off the label. People who have tried it, or seen others try, know how hard it is. Supposedly, once you have the label completly off and perfectly inatct, you are able to redeem this label for sex with a person of the opposite sex.
Woah! Chris just got himself a sex coupon, I wonder if Jessica has a double coupon day so I can get some action too.
That book your girlfriend makes you with sex coupons that when you try to use them all they she tells you to fuck off.
Me-"Yeah, can I use that sex coupon for one free BJ right now" GF-"Nope, fuck off asshole"