Examples include Ko-Ko being engaged to his ward Yum-Yum at the start of The Mikado and the Lord Chancellor almost marrying his ward Phyllis in Iolanthe. In neither case does there seem to be any objection to their getting married other than that the man is not the romantic lead. An inversion happens in The Pirates of Penzance with Frederic nearly getting engaged to his nurse Ruth, yet the age difference between them is here accepted as comical, and in any case Ruth would not have had guardian status.
Follow-up: how common was it for upper class European families to have a legal ward? Is young ward just a plot trope, or was it actually common? (I see this also in The Count of Monte Cristo and The Importance of Being Earnest.)
Follow-up question: how exploitative were these relationships in practice were these relationships exploitative in practice, and how did that relate to the depiction of these relationships in opera?
edit: I'm not satisfied with how I asked this follow up question originally, as I think it betrays my own bias. I'm not sure my edit is good, but it might be better.