I get it!
It just doesn't work that way, period, because pregnancy isn't like an infection, where someone's body just needs to be introduced to the germ. Rather, sperm cells are only one *part* of the whole process, and only one of several factors essential for pregnancy. They also can't go anywhere all by themselves. They lack that ability.
To find out more about how pregnancy actually works, if you don't know already, take a look here: Human Reproduction: A Seafarer's Guide
But if you already know the facts, more times than not, fears like this don't get solved with facts, because the fears themselves are based in something else, like, for instance, whatever it is that makes the idea of pregnancy so scary for you. For instance, one common situation with that is that people are afraid of being shamed, ostracized or kicked out by family, or even outright abused, so it's really THOSE fears a person needs to deal with to address THIS fear. Get what I mean?