Getting an STD from a toilet seat is very possible but also very unlikely because the STD can only live for a very short time on the toilet seat.
Also to get infected with an STD from the toilet seat the STD would have to be transferred from the toilet seat to your urethral or genital tract, or through a cut or sore on the buttocks or thighs.
So it's not likely that you would get infected with an STD from a toilet seat but if you're worried you can put down a toilet seat protector first or bring some sanitizing wipes to clean the toilet seat before sitting down.
You could also lay some toilet paper down on the toilet seat to protect you from germs as well as a possible STD.
But like I said it's very unlikely that you would get infected with an STD when sitting on a toilet seat.