Okay, first of all folks, I know there are already some questions about gaining weight.
The question in this case and the main difference with the others is on the conditions to achieve the same goal: Gain weight, lean mass to be specific.
I train kickboxing 2 times a week 2 hours each time, and it is a pretty intense and conditioning exercise. I swim two times a week as well. I have been trying to gain weight for a long time by taking protein supplements and increasing my calorie intake (eating 6 meals a day that are very protein rich). I am rather reluctant to do weight training because I have heard that you lose flexibility and that is essential in martial arts (I guess that is for another question). I am pretty ignorant on these subjects and would like some reference to texts about how to achieve lean muscle weight gain. I have read about the irregular fasting diet, but I am rather skeptical about it because I am mostly afraid of losing weight further.
I would like suggestions and if possible personal experiences in a situation like this one, maybe a healthy nutrition program (not drinking a gallon of milk and blah blah). I will go to my physician soon and ask him as well, but I thought maybe I would get some opinions here about it.
PS: I am 21 years old (almost 22), weigh 63-65 kg. (depends on the day, always fluctuating), and ~6ft.