I know this sounds gross but...
Your towels might have a yeast infection. Everybody's body has a natural flora of bacteria and yeast on the outter layers of our skin. Yeasts, also known as fungus, are very difficult to get rid of due to the way they reproduce and produce spores. The spores are kind of like dormant yeast that are enclosed in protective shells. They are so robust in fact, that spores are often found from pre-historic ages and once given the right conditions will bloom into activly growing yeast.
The right conditions to bring out the growing yeasts is a warm, moist environment. Like a nice thick towel hanging in your warm steamy bathroom to dry.
You need to wash the heck out of the towels, throw everything you got at em. Oxygen bleach, sttp, the hottest water the color can take. All of this is to try and wash away the spores, there is no way to kill them, you just need to rid the towels of them. Now that you have washed them away, you need to keep the new ones, which will get on your towels each and every time you dry yourself, from growing. The easiest way to do this is to hang your towles up in such a way that they dry quickly, before the spores can germinate. Make sure that your towel, epecially thick ones, are hung on a rack, in a single layer with lots of ventalation. Dont fold them in halfs or thirds before you hang them back up and dont hang them on a hook. They need to dry fast.
Our bath towels had the funk, but its gone now! Another thing you can do, although it sounds dreadful, is find some towels that have a small % of synthetic fiber like poly and that arent quite as thick. This will help them dry faster. Also, make sure your bathroom has adequate ventilation either from a window or an exhaust fan. If you go the fan route, make sure that the fan is sized to the room. Thre are guidelines as to how many times a fan should exhange the air in the room per hour. Most newer houses the fan the builder installed is way undersized for todays larger bathrooms. All it does is basically make noise. We replaced the 50cfm builder grade fan in our master bath with a 350cfm fan which is what the room size calculator determined we needed. We also installed a wall mounted electronic timer switch that allows us to hit one button and the fan will go on a 10,20,30, 60 minute countdown and turn its self off. This has a couple benefits, you will rid the room of moist air, and then not forget to turn it off. An effeictively sized bathroom fan, if left on, will suck your heated or cooled air out of the house, wasting money. This avoids that problem.
I attached a ventilation calculator to help determine fan size. I also highly reccomend the Panasonic fans at this site, they have proven themselves in our last two homes.
