How can I raise only my men-at-arms?

While I'm unaware of a way to raise only men-at-arms, it's relatively straightforward to (effectively) achieve just that:

1. Pause the game
2. Select your chosen Rally Point and press "Raise All". This will immediately raise all your knights, mercenaries, and Men-at-Arms.
3. Ctrl+Right Click on adjacent barony. This will order your Men-At-Arms to move out immediately and stop Levies from assembling.
4. Unpause. Proceed to do as you will with your Knight & Men-at-Arms stack.

As of 1.5 Azure

The 1.5 "Azure" Patch purports to bring a "Raise Men-at-Arms" button to make this even more streamlined.

Another option is to select a rally point and choose raise local army. This will raise all men-at-arms and only local levies, which number between a few hundred to a few thousand depending on how developed the local area is.