technically yes. herpes simplex is directly contagious and can occur on the buttocks itself so if you used a toilet relatively soon after another person who had active herpes simplex on their buttocks you can end up getting exposed on your buttocks. the herpes virus can remain viable on certain surfaces for hours so it is biologically possible. if you are female and have herpes on your genital/vaginal area it is not likely this part touched the toilet seat so the source was mostly likely direct contact with another person who has herpes.
can you get herpes simplex 2 from a contaminated toilet seat?