To those criticizing MC getting flustered when he's already had sex with his wife, even though I'm not married, even I know there are tons of new things spouses can explore together as they're married, and those new things bring new, memorable experiences that can spice up their marriage. Just because both of them have already done the deed doesn't mean both of them are immune to any other scandalous events or activities.
Obviously, MC has never seen his wife in a maid outfit, nor in a naked apron before this chapter, and she has never worn either of them up to then, as well. Of course, they'd get self-conscious stepping into something new and are overwhelmed with their own emotions to process it all. That's the best part in exploring it with someone you trust your everything with and not just some stranger.
It's honestly sad how people don't see the appeal in that these days.