Granted, some very confrontational Christians may contribute to this perception. In point of fact, though, there are many occasions when being for something automatically means that you are against something else---and there can be no middle ground.
The organization Mothers Against Drunk Driving is FOR responsible, unimpaired driving. Members of this organization will not congratulate drunk drivers, but will argue for safe, sober driving on the part of all motorists---and rightly so.
Or: Consider vocal environmentalists. If such people are for protecting the environment, they will speak out against whatever harms the environment. Such persons will not publicly applaud incidents of pollution being streamed into the atmosphere, nor will they endorse the dumping of toxic waste into rivers and streams. Devoted environmentalists will be Against such behaviors by speaking up For environmental preservation---and rightly so.
In Matt.12:30, Jesus declared: “He who is not with Me is against Me, and he who does not gather with Me scatters abroad." Jesus announced that whoever was not FOR or WITH Him was in fact AGAINST Him. He offered no neutrality with reference to Himself. Why should He offer neutrality? After all, the Crucified and Risen Savior gave Himself for sinners who place their trust in Him concerning their salvation, so that they will be With Him forever (1Thess.4:17).
Are you For and With Jesus Christ? Do you have genuine faith in Him as Lord and Savior? There is no middle ground.