STIs got their name because their primary mode of transmission is through sexual activity. Yes, there are some STIs that can be transmitted via casual contact, including scabies and pubic lice, but these parasites aren’t equipped to survive on smooth, slippery surfaces.
Scabies and lice can cling to towels, clothes, and beds, but not toilet seats. Bacterial and viral infections such as chlamydia, syphilis, HPV, and herpes, to name a few common ones, are transmitted via contact with infected fluids: genital secretions, semen, and blood.
Contact with infected skin can also lead to STI transmission. Contact with toilets, however, won’t do it. That’s because most pathogens that cause STIs can’t live outside of the human body for very long without deteriorating.
Source: DR Albert Donkor