Not because it's gross, but because you could be giving yourself your own HPV.
A recent study by The Ohio State University found that post-oral-sex-frenching is down right dangerous.
The findings also show oral HPV infections are, for the most part, sexually transmitted. People who reported engaging in oral sex were twice as likely to have an oral HPV infection as those who did not engage in oral sex.
However, there are clues HPV may also spread by kissing. Oral HPV infections were more common among sexually experienced people who had not engaged in oral sex than among sexually inexperienced individuals, a finding that is “consistent with transmission by other sexually associated contact (eg, deep kissing),” the researchers wrote in their study.
That means, whatever you've got in your junk right now is probably also in your mouth. Which mean it's time to wear condoms during blowjobs and use dental dams when going down on girls. Or not. It's just HPV. I'm sure it can be cured with some Orajel or something.