Hi!
We have a list of story tags that we use, and encourage our authors to use. When you input one or more of those into the search field for “Tags” your search should be filtered properly. If you want a story, for example, that is male/female pairings and incorporates oral sex, you would search for MF and Oral. The “and” means you want both tags to appear. If you wanted a male/female pairing but no rape, your search would look more like this: +MF -rape. If you don’t use any modifier, and searched for “MF Oral” your search would turn up ALL stories tagged as MF and ALL stories tagged Oral, so modifiers can help to refine the search.
As is pointed out in our search FAQ, you should navigate to the right subdomain before searching. The search page will allow you to further navigate to a category within that subdomain, such as Black Butler within the Anime subdomain. Every subdomain will have that ability, even single fandom subdomains like Harry Potter. Using that to further refine your tag search will give you better search results. If I search the Harry/Ginny category in that subdomain, and also use the tag search for Preg, I get only stories with that pairing and which also are tagged for pregnancy.
I hope this helps!