spaox, many Americans are under the impression that Brazilians are a sensual, uninhibited people who are much more open minded about sex, including sex between men. Many Americans find Brazilians exotic. However, since you've been in the states since you were a child, these may not be factors in your case.
With regards to uncut cocks, most American men are circumcised and many have never seen or felt an uncut penis. To most Americans, uncut cocks are "normal" and uncut ones look weird. (Kenyth's perspective makes more sense: uncut cocks are normal, cut ones are scarred.) Many American men and women believe that uncircumcised penises are dirty and that they look gross. Up until recently, I think many gay American men had similar attitudes. I've heard of men, both straight and gay, getting cirumcised as adults because of the negative attitudes towards being uncut.
Over the past couple of decades, gay Americans has been moving towards a more foreskin-positive attitude, and some have even developed a preference for uncut cock. For some, this preference is based on an objection to needless body modification (some would say "mutilation"), particularly when it is performed on infants who cannot give informed consent. For others, it's based on actually experiencing an uncut cock and deciding that having a foreskin really is better. For some cut guys, it's just fun to play with something different than what you've got. For me, it's all of the above.
Why are attitudes changing? It may have begun when gay porn like Bel Ami and Kristen Bjorn began featuring uncut models, as Matthew and invisibleman mentioned. The first uncut dicks I saw were in Bel Ami photo shoots, and I was fascinated. I thought they were really gorgeous, and for the first time I began to wonder what it was like to have a foreskin and to consider what I might be missing.
Also, in general, I think gay men are more open-minded than the general population, especially with regards to matters of sex and sexuality. Obviously we're more likely than straight guys to think about the variety of dicks that are out there and to be curious about dicks that are different from our own.
spaox, you say that uncut cocks don't operate differently from cut ones. Actually, they do function quite a bit differently. Kenyth does a good job describing the differences. As he pointed out, when masturbating, cut guys rub skin on the penis, creating stimulation through friction. The skin doesn't slide much, if at all. Unfortunately, friction can also cause irritation, often requiring the use of messy lubes. (I hate having to use lube. It's expensive and no lube is perfect. The ones that give the best slide tend to dry out the fastest. The ones that last long have a less sensual texture, etc. When you run out you try lotion or even vegetable oil as a substitute, but they absorb too quickly.) Friction on the penis and the need for lube can also be problems during vaginal or anal intercourse.