Because you are worried about contamination as well. Being grossed out does not always lead to worrying about contamination, unless you have a reason to worry that something is contaminated. Also, as you mentioned, you have some other obsessions going on here too, so I don't think it is just about feeling grossed out.

Feeling disgusted or repulsed may be more the end result of worrying that the body fluid is infected or contaminated with something, but what? A virus, infection, bacteria, something sinful or shameful, or something else?