This is a medium-risk practice that runs on consent: either of you can pause, slow down, or stop at any time — including the giver. If you're the one giving and you're not up for it today — hesitation about fluids or about the area itself is a real, manageable feeling, not a malfunction — 'not today' is a complete sentence.
Oral sex can transmit several STIs — among them HPV, herpes, syphilis, and gonorrhea — though the risks are far from equal. Herpes passed from mouth to genitals is now a leading cause of first-episode genital herpes among young adults in many high-income countries; HIV transmission via oral sex sits at the very low end of per-act risk; and trichomonas via oral sex has been documented only as rare case reports.
If either of you has an active herpes sore or ulcer — lip, mouth, or genitals — skip oral contact until it has healed. Barrier and protection choices are a medical question: this article doesn't teach protection categories, so take those to a clinician or a public-health source.
Pain, flinching, or bleeding is a stop signal, not feedback to push through. And a persistent change in odor — especially with unusual discharge or itching — is a reason for the person experiencing it to see a clinician. It is not a verdict on anyone's cleanliness.