How can you get your spouse to undress?

Solution 1:

Pickpocket her clothes before you marry, and she will never wear clothes.

Tested on the Xbox 360.

Solution 2:

Undressing

This is not really possible without mods.

There are a few different mods, and which one works best for you might take some experimentation. If you're on console, I'm not sure what the best link is, but my understanding is that most of console mods are also on Nexus, where you can explore the relevant category.

Sandboxing

Their behavior depends on the house they are in. With just the base game, your spouse will never leave the cell in which they are housed, so they cannot for example join you outdoors unless you bring them along as a follower. (Which might be a bad idea, as many of them are weak.)

Again, mods address this, and enable various options to customise the behaviour of spouses as if they were followers; Nexus seems to be the place to explore mods, and hope they are available for your platform if you're on console.