If you don't know about cock rings, well, the theory is that wearing one during sex traps blood in one's penis. Thus, one's erection is bigger and harder, and stays hard longer. They seem to work well for some guys, not so well for others. (And yes, straight men wear them too.) Personally, I just like the decoration factor. Cock rings are like push-up bras for boys' bits. Even the simplest leather strap, slipped under the scrotum and snapped over the shaft, brings everything forward and together so nicely.
First-timers, take note: Threading your cock and balls through a tight-fitting metal ring is tricky and takes practice. Don't try this on your maiden voyage. There are lots of other choices, though--leather or latex straps that snap, Velcro closures, stretchy ones that look like a cross between a rubber band and a gummy bear... and that's just the single-strap ones. Once you've got the hang of it, you can also get into multiple-strap cock rings, with straps that separate the scrotum from the cock, and sometimes push apart the balls themselves, inside the sack. Part of the charm of this is, as one pulls the skin tighter and tighter, everything becomes more and more sensitive. Delightful!
How tight you wear a cock ring is a matter of individual taste and comfort. I've heard that one should not wear a cock ring longer than 20 minutes, although I have personally seen folks wear them longer with no apparent ill effects. It's best not to fall asleep in them, though, so make sure you unsnap after sex. That caveat aside, cock rings are a low-risk, low-investment sexual toy. If you've got a penis, try strapping it up for your sweetie's Valentine pleasure.
