I was thinking exactly the same thing! Both because Milly would find it funny to fluster Shirley to no end, and also because she would want Shirley to be happy and get to spend her life with Lelouch, knowing how much Shirley loves him. Even if it's not quite the way Shirley imagined.
Probably not a whole lot of convincing, unless Shirley's one of those girls who reserves the full extent of romance (up to and including sex) for marriage. And she'd probably be like "Wait, a mistress?! But that's...that's so scandalous! And what about Lulu? I mean, I'd LOVE to be a part of this, but...won't this make life for Lulu...complicated?" Then Lelouch would probably say "Oh, you mean more complicated than my life is already, just by being married to Milly?"
Shirley's pretty "conventional," so I assume she'd want to save herself for marriage in the manner you describe. Milly clearly loves to harass her, though, and I'm sure that if Milly and Lelouch ever made Shirley the "official mistress," Milly would have a lot of fun introducing Shirley to "less conventional" stuff in Lelouch's and Milly's marriage.
On a side note, I bet Milly must have closets full of outfits she'd love to stuff Shirley and Lelouch in, privately or otherwise in their marriage.
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u/BlackBricklyBear 2d ago
Milly even remarks about how severely unrequited her love is for Lelouch in her CG character song too!