The source for the story is
www.eldiario.es, which is reputable. It's been taken up by the rest of the Spanish media.
A well know Spanish pornographer (known in particular for "bukkake") is currently charged (some months back) with abusing minors and human trafficking in conjunction with a Ukrainian "businessman". What have been published are chats between De Gea and Muniain and one of the protected witnesses (apparently a minor) in which she is asking De Gea what was the name of the second football player that was with Muniain when De Gea had arranged for her to service Muniain and the other guy (De Gea denies that it was him), and De Gea appears to be arranging for her to be with 5 football players on a subsequent occasion (he asks her if she can handle so many). The chats with Muniain are similar. The third player is rumored to be Isco, but last night on TV Eduardo Inda (an editor for the important scandal mongering newspaper El Mundo) dropped the name Javi Martinez too. But there is no public evidence related to those two, so this may just be mud slinging. The accusations are basically: 1. the women involved were minors (17) 2. arranging for prostitutes for others is pimping and that is a crime in Spain (although prostitution is not) and 3. the protected witness (and others) had apparently been forced to participate and was not paid.
It's the participation of minors and the accusation that they were forced that is causing the commotion.
Add to that the noise produced by elections and a European cup.
De Gea might very well not be on the Spanish side in a few weeks. Depends how this evolves.