she either wants to get pregnant by you OR she cheated on you, found out that she's pregnant and now wants you to creampie her so she can claim it's yours.
or maybe she just wants to get creampied. either way, i wouldn't do it but if you do, and she happens to get pregnant afterwards, get a paternity test and don't let her guilt trip you/shame/punk you into not getting one.
EDIT: also, always remember that the cost of a paternity test will ALWAYS be less than the cost of paying for a kid that isn't yours for 18 years (even more if the kid wants to go to college)