Marius is one of our more Problematic™
characters in canon, so take every discussion about him and his treatment of Armand/Amadeo with a huge truckload of salt and patience, bc there are some strong opinions out there.
Marius knew about Armand (known then as “Amadeo”) and Lord Harlech. He might have been jealous, but didn’t say so in TVA, only that he was worried for Amadeo’s safety:
“He has quite the reputation, your English lord, slamming down his knife on the board in any tavern he chooses. Do you have to consort with common murderers? You have a nonpareil here when it comes to those who take life.”
@auburnandamberangel, @sheepskeleton, @amadeo-child-of-the-renaissance, @faceofabotticelliangel, @annabellioncourt, @vraik, and @anton-mordrid would all probably have better answers for this… anyone is invited to reblog/comment and add their thoughts.
The main difference I see is that Marius gave Amadeo the autonomy and independence to make his own choices in this regard, and Amadeo was more rebelling against Marius in his fling w/ Lord Harlech than furthering his sexcapades education. It turned out that LH was bonkers, got all weird and overly possessive and wanted to kidnap Amadeo… it was a bad breakup.

[X, @cloudsinvenice said: I’m thinking Armand when he fights Lord Harlech, personally.]
There’s some debate about why Marius sent Amadeo around for all those sexual escapades and I always read it as part of Marius’ full-rounded education deal that he wanted Amadeo to have; to have experienced all the good things in life that can be humanly experienced, and to show him that physical intimacy doesn’t have to be the horrible experience it was when Amadeo was kept in the brothels. Not necessarily the best cure for a sexually abused child/teenager but Marius was probably thinking of it as exposure therapy:
Exposure therapy is a technique in behavior therapy used to treat anxiety disorders. It involves the exposure of the patient to the feared object or context without any danger, in order to overcome their anxiety and/or distress. Procedurally it is similar to the fear extinction paradigm in rodent work.