NvR2Thick..is basically right. The risk of infection is in the mouth and throat and not in the stomach. One thing to remember is to avoid brushing/flossing your teeth about 30 minutes before any oral sex as that often creates lesions where bacteria can infect. Also generally avoid oral sex if you've had recent dental work or any mouth sores, gingivitis or even cavities, which can also increase the risk of infection. Whether the man ejaculates into your mouth doesn't matter as much as you might think as a lot of the bacterial STIs basically live at the tip of the urethra in the penis and can infect from there and herpes and genital warts can be infected just from skin contact. Possibly HIV might be the only one that you'll be less likely to get without ejaculation.
Generally though oral sex is a less risky behavior than sexual intercourse, particularly unprotected sexual intercourse. Though just because it is less risky does not mean it is risk-free. This article from the Journal of Global Infectious Studies has some good information: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2840968/
TL: DR; Basically the better your oral hygiene is the less likely you are to get an STI through oral sex.
