Magnussen, being a total creep, pours an incredibly expensive alcohol into Sherlock's bottom by inserting the neck of a bottle deep into him and fucking him with it in the process (or afterwards if he uses something else to administer the alcohol). Sherlock is coerced into it (blackmailed perhaps). The whole experience is shocking for him, but I don't mind if his body betrays him, even though his mind hates it.

Later Charles also enjoys watching the show of intoxicated Sherlock being ill and absolutely miserable, crawling on the floor (possibly trying and failing to keep the liquid up his bum).

Because there aren't enough stories with Magnussen being a pervert he obviously is! I seriously don't understand why people won't exploit the canon. ;)