If you are serious about playing with fire then be prepared to get burnt. When will people realize that all it takes is one time to conceive and if it is not your intention to be a parent then by all means, please use protection. While I realize that the use of contraceptives is controversial, it is better to use this method of birth control than to bring a child into the world that you are either unprepared for or worse, you do not want!
I have stated in the past and shall continue to do so, abortion is a sin and therefore should not be considered as an alternative to carrying a child to term. There are many childless couples that would be happy and prepared to adopt a child, therefore if you made an error in judgment and a child comes along, please consider adoption rather than abortion.
What strikes me as insulting is when two people have an affair and become so wreckless that they refuse to use birth control methods but prefer to roll the dice and hope that nothing happens. The very act of adultery is in itself wrong, but to have unprotected sex (not to mention the health risks) is irresponsible.
Then there is the matter of the child that is being brought into this world by parents (usually one at this point) who never intended for this encounter being anything other than an affair. What happens to that child now? How are you going to explain to that child that what happened was a mistake and now you (and the child) must deal with it for the rest of your lives.