Why use a condom during oral sex?

Condoms don’t just protect you from the risk of pregnancy; they are also the most effective barriers against STIS

Oral sex is actually a pretty risky act when it comes to STIS and HIV infection, especially for the partner who is performing it. 

Therefore, unless there is a long-term monogamous relationship and are very sure about your partner’s health- meaning the both of you get tested for STIS and HIV at the beginning of the relationship and there is no cheating going on, the risk of getting and STI from unprotected oral sex is simply too high to ignore

The use of a condom or dental dam for oral sex might slightly reduce the sensation for your partner but it will drastically reduce the risk of getting an STI. 

Keep in mind that the person performing oral sex is the one who is at more risk of getting infected and therefore should have the final say about using protection.