As per Pubmed's website: "Certain enteric ailments are particularly common among homosexual men", including gonorrhea (which is probably the most common bacterial infection in gay men)
In addition to ___sis infections (that I have never heard of) not usually regarded as being sexually transmitted:
- shigellosis
- salmonellosis
- giardiasis
- amebiasis
Not to mention that many of the treatment medicines used can come with some serious side effects...
Bad link