You can not get pregnant by sitting on a toilet that happens to have a condom in it.
You can not even get pregnant by sitting on a condom directly. If there is semen inside the condom it is inside the condom, and even if some happens to be on the outside the odds of it getting anywhere pregnancy-inducing just by you sitting on it are pretty much nil.
Sitting on a toilet seat… that is above a toilet bowl… that contains a condom… and possibly also is full of the semen that has leaked out of the condom into the toilet water so that the sperm can swim around below you like a massive shiver of sharks waiting for the kill…
… is absolutely no risk at all.
Explanation for why you can not get pregnant by peeing above the danger zone:
Unlike dolphins and whales, sperm does not have the ability to leap from the water into your vagina.
Okay. Technically speaking, whales and dolphins also do not have the ability to leap from the water into your vagina either, but they do at least have the ability to leap from the water. Sperm don’t.