Why is it that everyone claims that HIV cannot be transmitted from toilet seats? What happens if there was infectious fluid left on the seats, like menstrual blood and vaginal fluid for example, which is a possible scenario in women's toilets? What if I come into contact with such fluids without noticing their presence there?

I don't know what toilets you've been in, but I've not encountered menstrual blood or vaginal fluids on toilet seats and have a hard time imagining how such fluids could get there, anatomically speaking. And even if these fluids were on the toilet seat, they would have had to have been deposited there very recently and you would have to have a fresh cut right above where the fluids are. This is why experts say not to worry about catching HIV from toilet seats.