For about 3 years straight this was the me & my ex's preferred method of birth control, not that we needed one with my family history of low sperm count and her endomytriosis and highly irregular periods. We never once had a pregnancy scare (except for that one time when the condom broke and we didn't know it), and it was absolutely never a problem where I would forget to pull out or she didn't have enough warning to get off of me in time. Communication will go a very long way towards preventing such 'accidents'. Self control, doubly so. This isn't for the weak willed.
That said, USE A RUBBER. We knowingly took the risks and went in to it with plenty of research and consideration. Again, USE A CONDOM.
At the opposite end of the spectrum, my current play partner is on the pill and still requires me to use a condom. I, of course, happily oblige.
So suit up before heading in, it's always better to be safe than to be sorry for the next 18 years
