Rory already linked to a pertinent discussion on Polyamorous Misanthrope, but in case you didn't click it I'll reiterate here:
Tubals and Vasectomies are NOT 100%, especially as time passes. Sometimes couples make the mistake of thinking , "I've had a vasectomy/tubal so we don't need to worry about unplanned pregnancy." Umm, no. It's far less likely, but it IS still possible. Just something to keep in mind.
Pan Female, Hinge in a V between my mono (straight) husband, Monochrome and my poly (pan) partner, ThatGuyInBlack