Part of the issue, I believe, is the “Universalist” in Unitarian-Universalist. That is, a large part of the religious background of the UU faith is intertwined with the idea of universal salvation, and that doesn’t synch too well with with preaching hellfire and eternal damnation.
I don’t see why UUs would intentionally try to keep a Christian out of their church, but I’m sure they would feel it appropriate to disagree with the person. That’s part of the reason that I have been so impressed with UUs (to the point of considering myself one over a relatively short period of time). Christians aren’t unwelcome, but they have to realize that the vast majority of UUs do not believe in judgement and Hell.
The lack of a set dogma allows a lot of flexibility. On the other hand, UUs are not going to be very receptive to Christians who want to convert them or to tell them that their beliefs are the only way to salvation. The entire point of UU is that exploring religious ideas and critical thinking are positive, and that – naturally – given this, people will end up at different conclusions about spiritual truth. If your philosophy disagrees, then UU isn’t really the place for you.
I imagine that there is an additional possibility – many UUs, in my experience, are ex-Christians. They do not want UU to become Christianity, as there are already quite a number of Christian denominations to choose from and they have already left Christianity (for whatever reason). Christianity and other religions don’t mix all that well.
This is not to say that there couldn’t be a UU with a major problem with Christians. I’m sure there are some. However, as an organization, I don’t think that UUs are anti-Christian by any means. After all, the Biblical tradition is a part of their faith (albeit a waning one in recent years).
If an individual came into a UU organization preaching fire and brimstone, I imagine that there would be a negative reaction from some individuals. However, I feel that reaction would be far, far less negative than if a Muslim came into a Christian organization in order to proselytize. While UUs have things in common with Christians – just as Muslims do – UU is not a Christian denomination (at least in the strict sense of the term) and while strict interpretations of the Bible are certainly recognized, they don’t necessarily jibe with UU beliefs. Still, UUs are about the most tolerant and open religious organization I’ve ever experienced.
Read the 7 beliefs that SisterCoyote posted – they’re about the closest to a creed UU gets to. However, I don’t think that a Christian who believes in hellfire and damnation could agree with all of these. Specifically, UUs encourage acceptance of the religious beliefs of others. Christians who believe that other religions are a path to Hell do not practice such acceptance, and hence, beliefs along that line would not really be Unitarian-Universalist.