I think you greatly missread, or misunderstood my last post.
Firstly, I don't just lift weights with my arms. My routine is very diverse and encompasses my whole body. I lift heavy, and eat big.
Secondly, why on earth would I stay away from lifting???????
Lifting has already produced desirable results in my game and also I might add, my life. I used to be that wirey 130 pound kid, now I have controlled strength, stamina, endurance, stability, etc.
On a side note, why do so many people consider lifting weights a bad thing? If you have specific goals, and a bit of intelligence, weightlifting can be the best thing that's ever happened to you.
What you are doing sounds perfect, and your weight is just right for a 19/20 year old. You can be one of those really athletic golfers of your generation.
Now what people normally expect when someone says they'll work out to build up mass, is what I did.
I went on a "cyber" diet when I was roughly your age (about 20 years old, 5'10.5" and only 150 pounds, but really thin), and a really aggressive lifting program.
I gained about 38 pounds and lifted really heavy weights. Bench 320, military press 210, etc, etc.
As I grew older, my mass increased only more. Mostly the marbled fat in between the muscles. It got to a point that I neede to go on a diet to get down from 239 pounds of unwanted weight.
Now that I am older, I wish I had not done it. Luckily, I did not damage any of my bones/tendons. I knew a few people on the same exercise program with terrible side effects.
I can't help but feel that if my arms were thinner, that my swings would be faster. Not that it needs that much more speed up to meet my golfing goals.
However, this is probably why most golfers feel that gaining weight is detrimental to your game.