How do you make whole-wheat bread less crumbly? -- (Without adding gluten)
I have found the water roux or tangzhong method effective for 100% whole wheat bread, as it makes the texture less dense, and therefore less crumbly.

Also, you may simply be using too little moisture overall. Now that I'm baking bread for a 2 year old, whose tastes lean slightly less rustic in bread than mine, I've rediscovered high-hydration loaves, which, when baked at a lower temperature than my beloved crusty, chewy breads, are pretty suitable sandwich material.

Another strategy, if you want to embrace the density of whole grain breads instead of avoiding it, is to try the strategy employed by dense German multigrain or pumpernickel loaves, which often employ moist sweeteners (honey, molasses or sugar beet syrup), sometimes additional moist ingredients (apples, carrots) along with various nuts and seeds that offset the grains, sometimes alternate grains like rye or spelt, and moderately high hydration. A picture of one example can be seen here: http://www.chefkoch.de/rezepte/1168871222955627/Apfel-Karotten-Mehrkornbrot.html. Then you get a fairly chewy but reasonably topping-friendly bread. (Most German sandwiches using this type of bread are remarkably minimalist in comparison to contemporary American preferences, sometimes meant just as a take-along "second breakfast", and may only have a slice of cheese or salami, some butter and mustard on thin slices). The combination of factors makes for a moister-than-you'd-expect, less-crumbly-than-just-grain, dense bread.