Swim trunks for men look like shorts but are made specifically for wearing in the water. Different styles of trunks have certain features, but mostly they are designed to be comfortable while swimming. Most, but not all, men's swim trunks contain a net lining that adds to the comfort factor.
The netted lining inside swim trunks is designed to cover the crotch area. It is typically made of polyester, and the holes in the netting are relatively small. Some swim trunk designs feature a net that is part of a larger garment lining, while others simply have the net in place.
The primary purpose of the net inside a pair of swim trunks is to provide support to the genitals while swimming. The netting hugs the genitals close to the body in a manner similar to brief-style underwear. This keeps them out of harm's way during physical activity. Swimming involves a good deal of leg movement; keeping the sensitive bits and pieces up and out of the way allows swimmers to focus on their technique and enjoyment rather than on whether or not they will injure themselves. Additionally, swimsuit netting prevents chafing by creating a barrier between sensitive areas of skin and the trunks.
Netting negates the need to wear underwear underneath swim wear. Netting is used instead of a solid fabric to improve the circulation of air, ensuring that the areas covered by the net don't stay damp. Netting also provides a smoother line under the swimsuit -- anatomical features are evened out, making fluctuations in size and location, as well as movement, less obvious to onlookers. This can be a beneficial feature for those concerned about wearing a swimsuit in public.
Not all swim trunks come equipped with netting. Board short-style swim trunks, which are cut closer to the body, typically do not. Try on swimwear to ensure a good fit; swim trunk netting that is too tight can cause pain in the groin and netting that is too loose provides little or no support. If the rest of the trunks fit but the netting does not, seek another size or choose another pair of trunks. Some swimmers find the netting uncomfortable or experience chafing. Look for swim trunks without netting or cut the netting out of swim trunks.