What Causes Erectile Dysfunction?

Hello, I believe that I am currently suffering from ED. having been examined by my GP and an Edocrinologist at my local hospital I have been advised that I am physically fit and healthy. I have not been in a relationship for approx 15 years and have recently discovered that I am bi-sexual. The partners I have met recently have all been other males and and now left wondering if physcological issues could be causing this problem? I had a strict upbringing where sex was considered dirty but as a 49 year old male feel I have the right to a private, persoanl life. Please help.