How can I force my desktop browser to never use mobile Wikipedia links (e.g., en.m.wikipedia.org)

You may use the Redirector add-on, available on Firefox and Chrome:  

This simple extension allows you to redirect urls using the power of regular expressions.

For example, you can have all mobile Wikipedia urls to open in desktop mode (this is convered in the tutorial below).

For documentation, visit the product home page for a tutorial with this example :

To set up a redirect for mobile Wikipedia urls, enter the following:

From: ^http:\/\/(.*?)\.m\.wikipedia\.org\/(.*) 
To: http://$1.wikipedia.org/$2

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As another possibility, you could also use the Firefox add-on Greasemonkey, available on Chrome under the name of Tampermonkey.

You could use this Greasemonkey script which redirects Wikipedia mobile to the desktop version :

// ==UserScript==
// @name        wikipedia_desktop_redirect
// @namespace   https://jspenguin.org/monkey
// @description Redirect mobile wikipedia to desktop version
// @include     http://*.m.wikipedia.org/*
// @include     https://*.m.wikipedia.org/*
// @version     1
// @grant       none
// ==/UserScript==

var m = /^(https?:\/\/.*)\.m(\.wikipedia\.org\/.*)/.exec(location.href);
if (m) location.href = m[1] + m[2];