Let's get one thing out of the way first; shunning has been practiced in the Catholic Church in the past. There were formerly two degrees of excommunication, vitandi and tolerati. This is not reflected in current canon law. According to the Catholic Encyclopedia...
A formally published or nominative excommunication occurs when the sentence has been brought to the knowledge of the public by a notification from the judge, indicating by name the person thus punished. No special method is required for this publication; according to the Council of Constance (1414-18), it suffices that "the sentence have been published or made known by the judge in a special and express manner". Persons thus excommunicated are to be shunned (vitandi), i.e. the faithful must have no intercourse with them either in regard to sacred things or (to a certain extent) profane matters, as we shall see farther on. All other excommunicated persons, even though known, are tolerati, i.e. the law no longer obliges the faithful to abstain from intercourse with them, even in religious matters.
St. Paul also mentions shunning in his letter to the Corinthians and it is worthwhile to consider his remarks. Notice the distinction St. Paul makes between outsiders and the brethren. He does not expect the Corinthians to avoid contact with sinners but he is very strict on how they are to behave towards one who has been welcomed as a brother then fallen into grave sin.
I wrote to you in my letter not to associate with immoral men; not at all meaning the immoral of this world, or the greedy and robbers, or idolaters, since then you would need to go out of the world. But rather I wrote to you not to associate with any one who bears the name of brother if he is guilty of immorality or greed, or is an idolater, reviler, drunkard, or robber—not even to eat with such a one. For what have I to do with judging outsiders? Is it not those inside the church whom you are to judge? 1Cor5:9-12
His concern is not that the Corinthians will somehow be tainted by exposure to the sinful practices of the world around them. One could hardly have been in a more corrupt city in all the empire. No, he is primarily concerned with those who had been welcomed as brothers, received into the community of believers and then behaved in a way that brought scandal upon the community.
When we speak of welcoming homosexuals into our churches we need to ask who exactly we are talking about. Clearly, if the Church is to be a refuge for sinners then any sinner needs to be welcomed with open arms. The notion that one needs to be pure to enter is nonsense. Those who are searching, confused, questioning, wounded all ought to find a welcome in our midst. Repentance ought to be the result of fellowship with us, not a precondition for it.
There are, however, people and organizations whose avowed purpose is to change Catholic doctrine from within. Organizations which represent themselves as Catholic but which approve of sin should be viewed with suspicion. Nowhere is this contrast more apparent than in the Courage Apostolate which supports Catholic teaching and Dignity which seeks to subvert it.