In the UK no it doesn't...
My still-in wife and me out for 3 years have 3 kids...and twice in the last few months we have had "accidents" which required the morning after pill.
In the UK, at the pharmacy, you have a little interview to make sure you know the score and my wife clarified with them that it doesn't allow fertilisation. The morning after pill in the UK does two things....stops an egg being released and if an egg is already present it won't allow the sperm to fertilise it.
Not sure about other countries or different types of morning after pill but in the UK it is safe for the dubs...