Wow I think i've seen this topic in 4 forums already.
Surrender, you need to think about this from a different perspective. This would be the equivalent of a guy wanting children and the girl not wanting them - the guy could intentionally put holes in the condom to try and impregnate his partner when he was making her believe she was protected because they were using contraception. How would you feel if your boyfriend were to deceive you? This isn't a great example, but what if your boyfriend had aids and he didn't tell you? What if he passed it on to you and you didn't know until you had testing done and got a positive result?
If you truly love your boyfriend, you'll take his feelings into consideration and not deceive him. Also, to the previous poster, sperm doesn't automatically die once it's exposed to the air. Sperm can live for up to 5 hours outside the body, so unfortunately for the unlucky guy, this girl could do something stupid and impregnate herself.
I had read on the news a long time ago that another woman had done this exact thing - saved her boyfriend's semen and put it on her hands and used it to impregnate herself. You know what happened? The guy immediately broke up with her and he sued her for causing him emotional disstress since she became pregnant from his semen and she was trying to nail him for child support. As far as I know, the guy won the case. Just shows that some guys will take no nonsense from women.