Robotic reversing vasectomies. The obvious fact is, one's testicular junk is just hanging there. It is, by far, the easiest method of birth control, you get at it and get at it once. However, even with plenty of access, the obvious plan is to cut the vas deferens. This isn't a good sounding idea for men (at least not all men). Surgical reversal is possible, though, expensive and not something you want to plan on having in advance.
This is where my invention comes in. Reversing vasectomies. Rather than cutting, why can't we use any other method of stopping something at A from getting to B? Clamping, removing, bridging, blocking, etc. I propose, for your favorable consideration, cyborg penis! Basically hook something to the vas deferens which prevents sperm from getting through, until you want it to. It wouldn't be hard to power something and switch the settings (perhaps even installing a password) through thin scrotal tissue. One could reverse their own vasectomies repeatedly. Turn it off with your wife, turn it on with your lover. Leave it off until you choose to have kids. Basically, it would be actual birth *control*. You could turn that sucker on and off at will.
This would have mass appeal. If you might sometime in the future want some kid, you could still get one while you sew your wild oats. It doesn't have the this is forever feel that snipping that cord does, and rather than breaking ones junk... it gives you awesome cyborg powers. It would be something men might want to have, just to have. Now that's an invention! The reduced number of unplanned pregnancies, simplicity in the realm of birth control, and easing the options for women (none of which are remotely as easy) are just little bonuses.