Yes there is such a thing as motorcycle underwear! Base and mid layers are not the first thing that spring to mind on the importance scale when thinking of what to don when you're about to hop on your ride, but layering appropriately can really make the difference, comfort-wise.
It is easy to understand why dressing in layers makes sense in cold weather, but base and mid layers can really make the difference in hot weather as well.
Leathers and nylon can get pretty sticky when the mercury rises above 80 degrees (F). A layer of soft fabric between the rider's skin and the leather can help, because the layer acts as an offset to keep the leather from resting directly on the skin, allowing air to circulate and preventing the sweaty skin from sticking to the leather. In fact, some of the motorcycle underwear specifically designed for hot weather use is designed with ribs to provide maximum offset. This creates "micro-climate" within the garment, allowing an insulating layer of air to both keep you cool or warm when appropriate.
Also many products are made from a type of fabric that wicks away moisture (draws moisture from the skin to the outer layer of fabric), which can also add to the comfort on those hot and sticky days.