Unkempt gas station bathrooms might be the site of your worst nightmares, but it’s highly unlikely (though not impossible) that they’ll give you a sexually transmitted disease (STD).
STDs can be caused by viruses, bacteria, or parasites. Only parasitic STDs like cra bs (pubic lice) or trichomoniasis have any real chance of being transmitted by sitting on a dirty toilet seat. And even then, the likelihood is extremely low.
Your genital area would need to come into contact with the toilet seat while the parasite is still on it, and alive — and toilet seats don’t provide ideal living situations for parasites.
Exercise a little common sense: Use a toilet seat cover, and don’t linger.