I'm not 100% sure about this, but isn't yaoi the explicit stuff and shounen ai the family-friendly stuff? So if there's a tag saying yaoi, you should already be expecting the sex scene and there's no need for the smut tag.
Smut is no longer a "genre" tag and now more of a general "this content contains:" kind of warning, yaoi should be tagged as smut. The only time sex is assumed is if it's marked as hentai
Shouldn't all yaoi just be blanket marked as smut then? I've always found it confusing when tagging stuff so I've mostly been just basing off MangaUpdates, though that doesn't really work very well since there's the content/format/genre distinction here.