I ran across an interesting paper. Among men treated at a San Francisco City Clinic who only reported receiving BBBJ's (no anal or vaginal sex) in the previous 3 months, 4.8% tested positive for chlamydia and 4.1% tested positive for gonorrhea:
http://cid.oxfordjournals.org/content/49/12/1793.full
The study was confined to gay and bisexual men who visited an STD clinic, so the percentages are certainly much higher than they would be for a typical group of heterosexual men. Still, the paper provides strong evidence for transmission of gonorrhea and chlamydia to the urethra through BBBJ's.
There's been some research in the United Kingdom, indicating around 2% of sexworkers there have unsuspected gonorrhea in their throats, and about 4% have unsuspected chlamydia in their throats. So if you're getting BBBJ's from providers, you are running some risk of infection, because a small percentage of them have STD's in their throats.
You can also get herpes, syphilis and NGU from BBBJ's. Here's a cheat sheet,
http://www.sfcityclinic.org/stdbasics/stdchart.asp
Anyway, this is perhaps something worth thinking about if you're married or in a relationship.
Transmission of chlamydia and gonorrhea to men through BBBJ
