That was funny, sweet, sexy and surprisingly sympathetic.
You just *know* in the Wizarding world, between potions, spells, charms, even curses, there would be ways for Snape or any wizard to perform but that he doesn't indicates either self-repression or a genuine need to give and not take (perhaps a form of atonement?). That Millicent not only understands and accepts this, but presents him as her chosen despite it and that it is Snape who brings Dennis into their relationship is very telling, indeed. Much food for thought there, which is one thing I really love about this exchange.
Setting aside the psychological implications, the concept is a delight--the lingering Slytherin and Muggle-born prejudice in their world, their acceptance of life as it is and making the best of it without complaint. I enjoyed this possible post-DH future and would love to see more should you choose to revisit it. :)