Why is it that some people won't set foot in another's sanctuary, even if it was for a non-prayer function? Can an orthodox person be in a reform style temple sanctuary for a lecture or concert? I understand possible discomfort with prayer services but sanctuaries are often used for other purposes.
Rav Moshe in his Igros Moshe EH 2:17 second paragraph he seems to make it clear that for davening it is for sure assur, and even when it is a wedding an Orthodox person should not go. This tshuva was regarding Conservative synagogues; I am guessing that all the more so this would apply to Reform.
I believe R' Moshe Feinstein has a responsum regarding an Orthodox rabbi performing a wedding at a heterodox synagogue, in which he says "your job as rabbi is to perform weddings, regardless of location", but I don't recall if he addresses the sanctuary-vs-social-hall aspect. Hopefully I'll find it later?
I don't have a citation (if somebody else does maybe they could edit it in, otherwise I'll keep looking) but R' Moshe Feinstein z'l rules that kefira (heresy) is the same as avoda zara (idol worship) with regards to the halachos of entering a place of worship, and that since reform and conservative reject many if not most of the Rambam's 13 ikarim (principals of faith), they are places of kefira and the actual sanctuary may not be entered.
Aside from this there is the issue of maris ayin (giving the appearance of wrongdoing). Even if there were not a specific law prohibiting entering the sanctuary, reform and conservative are not halachically legitimate movements and entering them as a religious Jew might give the false impression that their breaks with Torah are acceptable.
R' Moshe Feinstein z'l is no longer with us. This is an excellent question to demonstrate the dictum that you should have your questions answered by your Rabbi, not by a book. Of course, your rabbi will know of R' Feinstein's rulings, but he will also know of your circumstances, the reason for the proposed visit, etc.
A friend of mine, a black-hat from Monsey NY, was recently told by his Rabbi that it was permissible to attend the Shabbat Shareit bat-mitzvah (in a conservative synagogue) of my friend's niece (his brother's daughter). The Rabbi cited issues of peace within the family and not insulting my friend's brother as reasons to trump (in this particular case) the reasons for not attending.