Stealing another comment.

things like worms (although not an STD, but can be passed through sexual activity) can spread from an infected person to a toilet, to anything they touch, etc. theres also public lice which can pass in a similar mannar. you can also get herpes, HPV, syphalis and trichomoniasis. while herpes, HPV and syphalis cant survive long outside the body, there is a slim chance you can catch it via shared toilet. parasites though, like pin worm, public lice and trichomoniasis are relatively easy to catch through shared items and toilet seats.

trichomoniasis is a pretty common STD to be infected with, so if the guy has STDs, theres a good chance he may carry trich