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The method is very unreliable, because pre-ejaculate? may also contain (small amounts of) spermatozoa?. Also, not everyone is able to note the arrival of orgasm in a timely manner, and interrupting just at climax can be psychologically difficult.
The advantage of coitus interruptus is that it can be used by people who have religious objections against other methods of contraception, and that it requires no appliances. The disadvantage, as stated, is that it is extremely unreliable.