@KingRagnarok150 it's a dress, so it's a girl's style! It's just that Chloe has already been mistaken for, and reported to his dad to be, a boy whom the Duke might be sleeping with. So now when she shows up in a dress, it'll get reported to his dad again but she'll be mistaken for a boy in drag. The Duke knows this and just wants to mess with his dad and make him think even more that he's gay
since either the dad's a huge homophobe or wants him to get with a girl already for heir purposes or both lol and it'll just be a bigger blow to the dad to see that instead of getting with a woman like he's been trying to make him do, the Duke would rather be with a guy who dresses like one, or I guess as he would see it, his son prefers being with a "fake woman" which in this world is probably seen as creepy idk
(Tl:dr: if you're talking about the boytoy comment, it's just means she figures she'll be mistaken for a boy in drag and doesn't want to embarrass/cause trouble for him that way, but the Duke just thinks it'll be funny to mess with his dad a bit and wants her to be seen wearing it
which is why he wishes he could see his dad's reaction lol, boy might not have had a great first impression but at least he's good at trolling his dad)