Don't let it get you down. Americans react unpleasantly to uncircumcised men because 1) they aren't used to it and 2) there's an attitude that it's dirty and unsanitary.
In short, circumcision became common in America about 100 years ago as a cure-all. Medicine wasn't standardized at the time, and most scientists/doctors were also businessmen looking to cash in on their "discoveries". Before the FDA was created, people used to sell "patent medicines" which were poisonous drinks mixed with lots of alcohol that claimed to fix everything from headaches to ulcers...but when the law forced them to list their ingredients, patent medicine died out.
With circumcision, health gurus of the period (notably the guy who invented Kellogg's brand cereals) suggested that removing foreskins would prevent masturbation, which they thought caused unhealthy sexual problems. They studied mental patients in asylums and noticed that a lot of crazy people masturbate a lot--they thought the masturbating caused them to lose too much semen and the fluid imbalance made them crazy.
They hadn't figured out that semen and spinal fluid weren't the same thing. They were also influenced by strict Victorian sexual attitudes which gave most proper and elite people a negative attitude of sex. So circumcision was marketed as a cure-all for any number of health and mental problems, and became standard procedure in hospital nurseries. After a few decades, circumcised men were the social norm and the influx of Jewish immigrants fleeing Nazi persecution added to the acceptance.
So it's a cultural thing here, and it's changing. In the last 20 years there's been concentrated efforts to stop routine circumcisions on babies, and the foreskin is making a comeback (THANKFULLY!) and younger people who've been exposed to them tend to be accepting, but there's still a lot of opposition by people who still have the old-fashioned prejudices about it.
But don't let the pressure get to you. Just think of it this way: if you asked your girlfriend to get breast implants because you think small breasts aren't sexy and even disgusting, she'd slap you in the face and dump you. So therefore, if your girlfriend/boyfriend thinks you should get circumcised because she thinks it ugly and disgusting (and believe me, many of them do) then dump them and date someone else. You said it yourself, you keep it clean and you don't have problems, so "if it's not broke, don't fix it" as we say over here.
Don't slap them though...that'll get you arrested.