It could well be a question of general stereotype more than anything
I've found the following article on the net written by a webmaster of an anime forum and it should provide a better insight on the sexual and violence content "commonly" associated with anime.
It seems to me a ton of fans groan with the fansubbing and commercial release of each harem anime title. Females I know show such violent reactions that they categorically refuse to watch anything in the genre because they say it's stupid and sexist. They view harem anime as if it were the Anti-Christ.
This is a particularly difficult question to address for two reasons. First, generally speaking it's not the entire "harem anime" genre that's unpopular in America. It's only certain titles within the genre that American fans often seem to disapprove of. Second, I think the common contemporary American reaction to "harem" anime is a part of a larger reaction to any and all anime that contains sexual innuendo.
As its name implies, "harem anime" are shows built around the theme of one young man living with or surrounded by several young women. (The theme also works when the gender roles are reversed, but for the purpose of this response I'll stick to the more common single male and multiple females.) Examples of harem anime include Tenchi Muyo, Love Hina, Hanaukyo Maid Tai, Girls Bravo, Sister Princess, Ai Yori Aoshi, Mahou Sensei Negima, Kanon, Da Capo, Koi Koi Seven, and Maburaho. Especially among hardcore American anime fans, there seems to be a significant backlash against this genre of anime. Based on statements I've heard from fans, I don't think it's mainly a political or sociological motivation that causes a distaste for harem anime. I think many American fans are simply tired of seeing the same cliché setting recycled so often. I hear fans complain about a lack of originality in harem anime far more frequently that I hear complaints of propagating sexism. I can't say, though, that the distaste for harem anime is universal among American fans because titles like Love Hina and Tenchi Muyo are still very popular and best selling titles in America. Likewise, the Negima manga is a best seller in America. So I don't perceive a universal dislike of all harem anime. I find more often that a number of American fans seem to dislike only recent harem anime shows.
My theory is that the backlash against contemporary harem anime is part of a larger recent trend toward criticizing "fan service" in general. During the 1980s and 90s, gratuitous nudity and sexual innuendo in anime was embraced and celebrated by American anime fans. During the 1990s Protoculture Addicts Magazine went as far as publishing three "Shower Special" magazines devoted to just cataloging scenes of nude girls bathing in popular anime series. And more recently AD Vision went out of its way to have fun with " fan service" by including a "jiggle counter" bonus feature on some of its more gratuitous anime titles. But within the past few years, the American fan opinion of non-sexual female nudity in anime has changed from one of playful appreciation to one of shame and abhorrence. I've seen numerous reviews that consistently cite Popotan, for example, as a show with unusually complex story and character depth ruined by excessive exploitive nudity. I think there are two contributing factors behind the contemporary objection over exploitive female nudity in anime: the rising number of American female fans, and the increasing mainstream exposure of anime in America.
As anime has become more recognized in America especially within the past five years, an increasing number of female viewers in America have begun watching anime. Rather than considering female nudity in anime a celebration of the beauty of the female form, or recognizing "fan service" as an innocent method of gratifying male viewers, female American anime fans and those they influence appear to have politicized anime with a socio-sexual agenda. Female nudity in anime is now considered sexist and shameful because it reportedly fetishizes women as sexual objects. Such a narrow view, though, doesn't allow for the possibility that competent, independent women illustrated nude in anime may reinforce a pro-feminist ideal that women can be sensual and beautiful without becoming mere objects in the service of men. Expanding this political stereotype, I think that male and female anime fans in America now often denounce any semblance of sexual exploitation in anime as a method of validating their own feelings of inferiority. I suspect that many contemporary American anime fans subconsciously feel insecure about watching "cartoons," so they feel a need to separate themselves from sexist, childish animation by denouncing objectionable elements in anime. The theory may be, "It's okay for me to watch cartoons because I only watch intelligent, mature cartoons; not ones full of gratuitous, unintelligent ones full of sex and nudity." But ironically, while there does seem to be a significant backlash against exploitive female nudity in anime and manga, the yaoi genre is increasing in popularity in America exponentially. I find it personally rather hypocritical that female nudity is now frowned upon by many American anime fans, yet male sexuality in anime and manga is not only accepted, it's the fastest growing trend in the American anime community.
So, if I'm correct, much of the contemporary backlash against harem anime among American fans isn't exactly a reaction to harem anime itself; it's a rejection of sexual innuendo. The reaction to harem anime among American fans isn't universal, and doesn't apply to all shows. But a significant percentage of American hardcore anime fans seem to be rejecting contemporary harem anime as a reaction to an over saturation of the theme, and as a method of positioning their own status as anime fans. Many fans are tired of seeing the same basic plot over and over again. And many of these same fans feel a need to distinguish themselves as discriminating, cultivated viewers unlike the typical, cloddish viewer that enjoys lowbrow entertainment.
For what little it may be worth, in my personal opinion, anime series are intended to be entertaining, and therefore should be judged on that basis. It's fair to point out an anime that's derivative, and it's responsible to point out an anime that advances a harmful gender or sexual stereotype. But the fact that an anime happens to include nudity or happens to be based on a cliché doesn't automatically make that anime bad. Prejudging a title based on appearances or superficial characteristics denies the original artists any opportunity to exercise their creativity. Even a cliché can be interesting and appealing when its presented in a unique and creative way.