There are 3 red light districts in Bangkok:

• Soi Cowboy - BTS Asok Station or MRT Sukhumvit Station
  
• Nana - BTS Nana Station
  
• Patpong - BTS Sala Daeng Station or MRT Silom Station
  

All 3 are really easy to get to, especially Soi Cowboy and Nana which are in easy walking distance. To get to Patpong - simply take the BTS or even the MRT if you are closer to Soi Cowboy.

Since Soi Cowboy and Nana red light districts are so close to each other I recommend staying in a girl friendly hotel in those areas for convenience.

Is it easy to travel to the red light districts in Bangkok?
It is very easy. All you really need to get to all of the red light districts are the BTS or Sky Train system or the underground subway system called the MRT. Bangkok's train systems are very efficient, affordable especially if you buy a multi-pass card which I recommend for those staying in Bangkok for longer periods. Just remember that both train systems operate from 6am to 12am daily.

Since the majority of adult entertainment establishments close around 1am to 2am you will most likely have to rely on hiring a taxi if where you're staying is not near by.