I am not a lawyer and don't have the expertise to give legal advice. I'm just summarizing what I've read.
Obscenity law is usually applied to materials far more sexually explicit than the library books to which the WBC4SL objects. To meet the definition of obscenity, the material would have to be prurient, patently offensive, and without serious value. Importantly, all three of these conditions must be met for an item to be found obscene.
Of these three, the patently offensive condition is probably the easiest to meet, since it's very much a matter of opinion. The prurience requirement is also a matter of opinion, but it isn't only a matter of opinion. There is a minimum level that material has to rise to before opinions matter. The important detail is that, by definition, a simple frank interest in sex and sexuality is not prurient, and that there has to be something unhealthful about the material.
The third condition, that of lacking serious value, is very difficult to meet. Any literature, teaching materials, medical works, and the like, would almost certainly be considered to have serious value, and therefore could not be obscene. It is important to note that while the prurience and patently offensive requirements are a matter of local community standards, the requirement of lacking serious value is not. Obscenity law says that serious value must be evaluated by the standards of a "reasonable person," a requirement included in the law as a deliberate limitation on the ability of local community standards to define anything they don’t like as obscene. Interestingly, the petition circulated by the WBC4SL excludes this aspect from their definition of obscenity.
In the unlikely event that the West Bend situation actually went to court, there would likely be a legal argument about the boundaries of the community that must be included in defining the standards of offensiveness and prurience. The library does not just serve West Bend. City budgeting materials (available online) indicate that there are some 50,000 library cards outstanding, but give no data as to which communities those card holders live in. Library policy (per the library's website) says that anybody can have a card, regardless of residence. Also, if I correctly understand the city's 2009 budget report, about 40% of the library's budget (roughly $635,000 out of 1.5 million, on page Lib-10) is paid for by Washington County, with the stated intent of serving county residents who live in areas without a municipal library. The same city budget report states that "the library will extend services to all residents of Washington County," (p. Lib-2). It is probably also significant that county administrative offices are located in West Bend. Most likely, applicable community standards would be those of the county as a whole, not just those of the municipality of West Bend.
By legal definition, such community standards are set by neither the most nor the least sensitive individuals, but by some kind of theoretically average resident. In some actual court cases, community standards were not left up to mere assertion, but have been measured directly. Defense attorneys have sometimes subpoenaed records about which websites local residents were visiting, what local adult bookstores were selling, or how many residents were watching adult pay-per-view on cable TV. This has made it very difficult to assert that community standards are as conservative as some claim them to be. Of course, none of the West Bend library materials come close to the level of sexual explicitness involved in those cases. Since Washington county cable television service includes the Discovery Health channel or MTV, the library materials are safe.
Another detail of obscenity law overlooked by the WBC4SL is that each work must be evaluated as a whole. The "offensive" excerpts they include on the Wissup blog are an effective tool for inflaming public opinion, but would be of little use in a courtroom. I am given to understand that the one exception to this has to do with child pornography, which allows any content to be evaluated without regard to the whole work. That area of pornography law is newer, so court precedent has not defined it as clearly as the broader obscenity laws. But here again, it is difficult to imagine that any of the library materials could fall into this category (much of child pornography law seems to hinge on whether there is a prurient depiction of an identifiable, individual minor).