You could eat your cum. I wouldn’t say its good for you or bad for you so i wouldn’t go out of your way to eat it, but your own cum won’t hurt you.  You could spread an STD from one part of your body to another.  If you think you might have an STD, you would still want to get tested (and possibly treated) no matter where in your body (genitals, rectum, throat) the infection is found.  It IS possible to get an STD from someone else’s cum.  For example, if you are giving oral sex, you could get chlamydia in your throat