Nudity serves a utilitarian purpose, but it also exposes all of your secrets that are normally kept hidden by your clothes. You're caught in a weird catch-22 where you want to do all the fun stuff that comes with being naked, but you're slightly afraid that being naked will make the other person not want to do them anymore. It's probably in your head, but that doesn't mean you don't still feel a little self-conscious.
You're not nervous about them seeing you naked anymore but....well, you still want them to like you. That's why you let it all hang out, but you make sure to do everything you can to make the it that's hanging out look its best. You suck in your gut, clench where you can, and perspective tricks that would impress Peter Jackson. You'll show them what you look like eventually, but for now you wanna put your best, naked foot forward.
Once you've humped enough to get over the hump, you start to feel comfortable enough in your own skin to just enjoy the nudity...and boy do you enjoy it! You have someone that you get to look at naked all the time! You have someone who wants to look at you naked too! You're basically living out your every teenage dream! It's awesome!
After a while, the initial excitement of nudity fades, but it's replaced with something even better...comfort. Once you've seen someone naked, their titties and/or dicks start to feel no more different from their hands and/or kneecaps in certain situations. Nudity stops being an event, which means if you and your significant other don't feel like wearing clothes...well y'all can just not wear clothes. It's pretty cool!
Once all hang ups are tossed out the window, you get to a point where you don't hold anything back and.... Well, let's just say, you start to remember why people think nudity is gross again.