It's yuri? How can they have sex if it's shonen? (Shonen manga isn't allowed to have sex in it.)

Perhaps you should read up on the definition of yuri.

Well you're wrong on that first part. There's sex in some Shounen Manga. Always comes as a surprise mind.

The Shoujo Ai tag tends to be used for subtext and the Yuri tag for actual text. though the line it a bit shaky sometimes