If a man has ejaculated on the floor of a shower, and then you go take a shower and sit on the floor, you’re not going to get pregnant.
Drains are not the Super Sekrit entrance to your vagina. When the semen slowly swirls down the pipes into the sewer system, it does not do a whole chutes and ladders thing to come back and fertilize you later.
Semen that has gone down the shower drain also doesn’t grow up into giant sperm that haunt the sewers and later attack by climbing up through the toilet. That only happens with alligators.
Explanation for why you can’t get pregnant by sitting on a shower floor where semen once was:
In order to think there was some possibility of pregnancy from sitting down in the shower, you’d have to assume that the semen was still sitting on the floor and hadn’t been washed down the drain.
Then you’d have to assume that the sperm hadn’t been killed by exposure to soap and other things that sperm don’t generally do all that well in.
THEN you’d have to figure out some way for them to climb up into the potential space that is your vagina – which is not actually a gaping hole waiting to vacuum up any potential sources of impregnation – and from there through your cervix to where an egg happened to be waiting to be fertilized.
It’s really not worth worrying your pretty, clean little head about. There are many nasty things that can live on a shower floor, but they’re not going to leave you carrying any new lifeform that isn’t microscopic.