The vast majority of STD’s are not transmitted through saliva, and the small few that are only do so in specific circumstances. The chances of catching an STD by drinking from someone else’s glass are negligible to pretty much non-existent.
A tranny with HIV could spit into your mouth, and you’d be fine. Even with bleeding open wounds in the mouth, the odds are extremely low.
It’s one of those age-old excuses for when you’ve been banging hookers and your wife wants an explanation for why you have an STD. Drinking from someone else’s glass, sitting on a dirty toilet seat, swimming in a pool, etc.