Sperm can go through thin fabric and underwear, and there are known cases of pregnancy resulting from situations like this where there was no intercourse.

However, for pregnancy to result from the exact situation described in this question, the semen would first have to defy gravity, and then penetrate 3 layers of clothing, not to mention 2 sets of labia. Almost impossible! But never underestimate a natural system that is designed for success. After all, we would not be here discussing this if it didn't work as well as it does.

All the same, it only takes one sperm to fertilize the egg, though it takes several to help penetrate the egg's outer shell: better to be safe than sorry!

Even if sperm does manage to make it to the egg (most will not and over 50% of sperm is unusable meaning it has some sort of problem that prevents it from making the successful journey), the egg is protected by layers of mucus - the zona pellucida, the corona radiada, and the cell membrane. A sperm cell motor function is powered by the mitochondria in the midpiece which uses the fructose secreted by the seminal vesicles as an energy source.

It takes a group of sperm cells to penetrate all those layers. A single sperm cell would quickly run out of energy before it could make it past the first layer.