A search of hundreds of sites indicates that the clothing recommended for avoiding UTIs would allow for freedom of air movement. Conditions that favour bacterial growth include the warmth and moisture that results from wearing tight clothing and synthetic materials.
Think COOL and DRY... Wear cotton underwear or gussets-- nylon and other synthetics will not allow heat and moisture to escape as well as cotton. I personally believe that ORGANIC cotton is superior since cotton, if it's not organically-grown, is one of the most heavily pesticide-sprayed of all crops.
Recent information suggests that cotton clothing (not underwear necessarily) bought at consignment stores and thrift stores is likely going to have been washed quite a bit and have most of the pesticide residuals removed-- this is particularly important for kids' clothing. I would also suggest that using a pure biodegradable laundry soap will eliminate other related problems with UTIs (*created by detergents and chemicals in laundry cleaners).
Some recommendations are:
Healthy weight loss and continuation of a good weight maintenance lifestyle, such as a whole food plant-based diet, means that there will be more opportunities to find attractive, comfortable loose-fitting clothing to wear.