For anyone who is still a virgin this might not be the answer, but for anyone who is or has been sexually active, this may be the cause. It's called Trichomoniasis or Tric.
Also, Trichomoniasis is very difficult to detect in men. So, it is possible to have been tested for STDs and tested negative, but still have Tric. Luckily, it is treatable with antibiotics.
In hope this helps at least some of you.