It is difficult for a woman to feel sexy as a man, and I'm sure a man likewise needs to feel manly to feel sexy.
It's all in the feedback, much of which comes from within (as we learn to see ourselves for who we truly are,) but the feedback from without is a matter which can't be ignored. The way people talk to you and treat you, the image in the mirror, blah blah blah. When these things say "man" yet your brain says "woman" it causes feelings of dissonance. When it comes to amorous times, if your partner is coming on to you as though you are the wrong sex, well for me it made the whole deal an experience I would avoid as best I could possibly manage.
As my mind and body have started to become more in tune with my internal gender, I have begun to see everything in the world ...differently. It's so similar to the first time I put on glasses and suddenly I could see the blades of grass in the lawn. I remember going, oh grass doesn't look like carpet ..weird.
I haven't even got very far yet and I have already noticed a lot of changes in my sexuality, as in I'm beginning to realize it exists. Changes is the wrong word, emergence is better.
For an FTM, Testosterone should not only help with the image issues, but also the chemical itself has the specific effect of increasing one's sex-drive. Surely there are no guarantees, but it would make sense taking the hormone would probably help awaken a dormant sexuality, if it is there.