Even if she's on top, and some semen trickles out, chances are just enough sperm cells could make their way up into her reproductive tract to result in a pregnancy. Bear in mind, the female body is built to get pregnant. At the right times of her cycle, there are various physiological mechanisms to facilitate sperm travel up to an egg (as well as mechanisms to bring the egg to the sperm). Nature -- whether you want it to or not -- wants to cause pregnancy.
If you are going to engage in "dry sex," you should still wear a condom to prevent unwanted pregnancy and infections. And if it has been less than 120 hours since the sexual encounter in question, the woman can take emergency contraception to prevent pregnancy. Otherwise you would have to wait and see if she gets pregnant.
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