Casting female singers in male roles is rather common in opera, and it's done for good reason. I came across this concise and well-thought explanation by mezzo-soprano Katharine Goeldner, who was Arizona Opera's Orfeo in the company's latest production: trouser roles.
Lyric Opera Baltimore's staging of Gounod's "Faust", coming up on April 20 and 22, is a co-production with Arizona Opera, which staged the opera back in November. It's another updated staging, but images from the Arizona performance that I've been seeing in my searches look promising. "Faust" was my introduction to the old Baltimore Opera Company around 1989/1990 (with the great Jerome Hines as Mephistopheles), so maybe I can make it my introduction to LOB.
And there's a trouser role in "Faust", too. Siebel, a student of Faust who also has a crush on Faust's object of desire, Marguerite, is a young man usually sung by a soprano rather than mezzo. Now and then, you'll find this role taken by a tenor, but, as Goeldner explains, a female voice makes sense in a portrayal of a young man or adolescent boy. (Actual boys singing these roles usually doesn't work, because they don't have the volume and stamina to perform an extended role in the opera house.)
Faust at LOB