You're not going to like this answer, but that's because I think the premise is flawed. Why is homosexuality given a standing all its own? What's wrong with the other hundreds of works and confessions by Reformed thinkers regarding sin?
The question is not one of legality, but one of morality. Speaking from a Reformed Baptist perspective (I know, not truly "Reformed"), such individuals would be welcomed as any sinner would. They may attend services and worship freely and should be unharried in their attendance. They are to be treated lovingly, as any other individual would be treated. They must be presented the true gospel, God's grace apart from works of the flesh, just as any sinner would.
However, the union is not recognized as a sanctified union before God (I don't feel the need to defend this here. It's worth a question all its own) and as such, the couple is to be viewed as two individuals. They should not be considered for any counsel, nor should they be encouraged to partake in the ordinances so long as they willfully consider themselves to be in a union.
Such a couple should not be made an example, and at the same time teachers must not shy away from teaching the hard truths in the scriptures that a homosexual union was not given as an object lesson of Christ's work but rather a singular heterosexual marriage is (divorcees who have remarried raise another question, too).
Regarding the legality of the practice and the fact that the union is between two consenting adults, the question is functionally the same as that of prostitution (imagining the church is in a location where such practice is legal) or heterosexual cohabitation apart from marriage. If the convert stops uniting himself with a person who is not his wife, then he is demonstrating the beginning evidence of the Holy Spirit's work in his life and should be treated as a brother (with struggles) and gently examined over time for trustworthiness of the faith. However, if he demonstrates no attempt to stop this practice, he should be treated as one who is not a convert but rather one who needs to hear the gospel.
It has been asked "What of homosexual couples who bring children to church? Should they be told to end their legally-separate union?"
First off (and this is the Baptist in me coming out), it is not the church's or church members' prerogative to be telling people what they should and should not do in their personal lives, apart from expositing the scriptures. If a homosexual couple is dependent upon each other financially and also has children, this is something of a unique situation. It is unhelpful to tell them "split up and never see each other again, and give your children up for adoption... again."
Yet we can not shy away from the truth. Speak the truth from the scriptures as you arrive at them. Do not make an example of these people. Pray for them. In God's perfect timing, their eyes will be opened and their minds will no longer think in futility. Incidentally, this is always the answer. All of the other things will work out so long as this precept is first.
If they want to partake in communion or Baptism, it must be explained that their presence in the church is appreciated, but they cannot take the sign of identification with Christ while they have no interest in repentance of their most obvious and public sins. Additionally, one cannot be in communion with the church (through the supper) while in the same position. If they insist on taking communion, they are guilty of profaning the body of Christ and should be repeatedly, gently asked not to take communion and be given an explanation as to why.
If this is enough to push them out of the body, I'm ok with that. Every effort has been made to be inclusive as far as the scriptures permit. It's a bad situation, and either it will turn biblical or it will not. There's not really a lot of middle ground. We must seek God's approval above man's, even if it means losing those who attend a church.
The edit in the question is applying a context that the passage does not warrant. Being circumcised is not a sin, nor is being uncircumcised. Being married is not a sin, nor is being single. However, a homosexual union is sin and is thus not applicable to the passage. One verse in the section sums it up nicely: For neither circumcision counts for anything nor uncircumcision, but keeping the commandments of God. (1 Corinthians 7:19 ESV)"