@kholl83:
It is an arbitrary but very well-established tradition among western manga fandom (certainly not unique to Mangadex, for instance MangaUpdates uses the tags this way), that "yaoi" refers only to male/male gay stuff with explicit sex scenes. All the rest of BL is "shounen-ai", as this is (correctly) tagged.
It's not like I have no
idea what social dynamics have lead a bunch of sweaty nerds to come up with such a system, whilst having "yaoi", but not "shounen-ai", function as the blanket term for the whole genre colloquially... >_>; Though I'm not quite sure from where Mangadex gets so
many users unfamiliar with the first but not the second of these terms, like yourself, compared to its predecessor sites. Regardless, I believe there's been talks of ditching the whole "shounen-ai/yaoi/shoujo-ai/yuri" thing on mangadex and moving to altogether different tags.
(Do note that these definitions for these words are considered pretty much made-up by the western fandom. But for better or for worse, we've been using them for decades. If you want the modern Japanese term, stick to "BL", for "boy's love", and yes it's beautiful and ironic that their term is also, likewise, questionably-appropriated English.)