Sexiness is playing to an audience, so as you learn what your audience likes, you can choose to emphasize those traits and be sexier. Some people find things sexy that we don’t have a lot of control over so those can’t be changed, but can be emphasized or minimized.
For example if my wife (my audience) thinks Vin Diesel is sexy. I can’t suddenly beef up, drop my voice three octaves and star in action flicks. Luckily she also thinks confidence, a genuine smile, and someone who makes her feel secure is sexy. Those I can handle.
So it can definitely be learned, but you can’t please everyone and you can’t be all things. Emphasize your strong points and people attracted to those will find you sexy.