You can not get pregnant from sitting on sperm.
You can not catch sperm from a chair. You can not catch it from a stair. Unless you sit upon a man* there won’t be sperm inside your can.**
Explanation for why you can’t get pregnant from sitting on sperm – and a big thank you to the numerous people who searched for our site using variations on that question and gave me some much needed inspiration:
In order to get pregnant, the sperm have to reach the egg. They’re good swimmers, but they’re not sneak thieves – if you sit on them, there’s no good way for them to stealthily climb up into your body, slide through your cervix, and have a hot date with an ovum. They don’t, after all, have rappelling gear, and the sperm not only have to get up into your vagina they have to make it into your uterus!
*and by “sit on”, I mean “have sex with”
**and by “can” I mean “vagina” – a word which rhymes with absolutely nothing useful.