I understand what you're saying, but that is a different issue. I agree that there's no issue with merely having a conversation, including in a discussion group, about theology. That should be encouraged as following the Great Commission.
The issue is one of witness and comes in the form of indiscriminately naming the forum "Lutheran," and then presenting non-Lutheran ideas as if they were Lutheran or could come under the big tent of something loosely called the Lutheranism tradition. The problem I see is that the point of the Confessions is to delimit what belongs in apostolic church, which Lutherans in those very same Confessions have said is a matter of right teaching. When we share a forum with folks who have abandoned the idea that the Confessions set the baseline for what is the teaching of apostolic church, we publicly abandon that standard as well. What I'm saying is, by having a common forum with them, we are saying that fidelity to right teaching is not essential for unity, which itself contradicts our teaching.
In short, quantenus Lutherans are not Lutherans, and quia Lutherans who share a forum with quantenus Lutherans are not making that point crystal clear, and are not abiding with God's will that His Word not be mixed with the teachings of man.
Now, I may not have considered everything, but it seems to me, to be true to ourselves, we ought to consider:
1. asking for our own forum, where we make clear (perhaps in a "sticky") the Confessional Lutheran idea of what the apostolic church is on the one hand, and what a heterodox church is on the other, and that we cannot share a forum with those who do not promise to uphold the apostles' doctrine (as set forth in the BOC, and ultimately, the Bible).
2. perhaps even that is insufficient. Perhaps it is necessary to have no Confessional Lutheran forum on CF, because those who do not visit it will only see another menu choice rather than the apostolic church. Maybe the solution is to ask that TCL forum be called the ELCA or quantenus Lutheran forum (though that seems impractical) and that we all use the space under our messages to explain that we cannot have our own forum on CF and include a link to a Confessional Lutheran site like CF but dedicated exclusively to Confessional teachings.
As a member of the LCMS, I realize that the WELS, ELS and others believe they can have no fellowship with me. A common forum might be a problem for them, but it isn't for me, since it is they, not the LCMS that refused fellowship. To the extent that the LCMS is itself heterdox either because of false fellowship principles or bad praxis is a related but separate issue that publically concerns only those outside the LCMS (though it privately should concern every Christian within the LCMS). Thus, as far as sharing a public internet forum is concerned, that is really only an issue for Confessional Lutherans outside the LCMS.