Just found out my high school sweetheart and long time bf is attracted to, and has had sex with, a transgendered male (male to female)...if I got that wrong my apologies...

This weekend while watching porn with my beau he suggested we watch a transgendered female video, thinking nothing of it I agreed and he seemed to enjoy the video quite a bit. Afterwards he shyly confessed that he was attracted to transgendered females. I found no real problem with it, until today when he confessed to me he had sex with a pre operative MTF a few weeks back. Now, I knew of his sexual escapade but from what he told me I had assumed he had been with a woman, since he had never mentioned his attraction to transgendered females before. When he told me that the person he was with was in transition to being a female I was shocked. I didn't know how to react. I cried, feeling betrayed because he had lied to me but I knew I couldn't push him away. I pulled myself together and hugged him and let him know that no matter what I'll always love him and be by his side. I know I have to be understanding, but at the same time I myself feel hurt. There have been many times when I have felt like I'm not able to give him something, and now I kind of know why. The reason I come here is to seek advice on how to help my bf feel comfortable with who he is and what I should do to be more understanding. Part of me feels betrayed yet I know that I can't feel anything negative about this. Has anyone else gone through a similar experience? If so, what type of advice can you offer? EDIT: Used the correct term; before this weekend my bf had never mentioned his attraction to these type of women. My assumption in him being with a natural born woman was genuine due to no prior knowledge of attraction to another gender.