Why is there no option for USB INTERNET in WIRELESS & NETWORK SETTINGS on my Nexus 5?
In Android 4.4.x you should be able to find the USB Internet option under:

Settings --&gt; More... --&gt; Tethering &amp; Portable Hotsport --&gt; USB Tethering




If you want to later on configure your network sharing settings so that your device can benefit from the shared internet connection, please read this stack answer.