Well I see several problems with your reasoning, both internally (within the reasoning that has resulted in your own belief) as well as externally (within the reasoning you're using to try to convince people to adopt your belief).
First, the percentage of the population that is gay/lesbian/bi changes based upon who is reporting the figures, as well as who is doing the research. The "New York's Gay City News" argues that it's closer to 20%, while conservatives tend to place the percentage lower.
Also, you make the assumption that law schools only give preference to "minorities" that are underrepresented in their schools or other schools. This is irrational. It's clear that Boalt gives preference to women, even though there are plenty of women in law schools in general, especially in theirs.
Furthermore, law schools often over-represent minorities that they give preference to (blacks, hispanics, sometimes women), with respect to their frequency in the national population as a whole (i.e. schools with well over 20% black, etc, while the % of them in the population as a whole is far lower). So the argument that there is a higher percentage of gays in LS than in the population (that they're overrepresented) as a whole is immaterial, even if it was established that they were overrepresented, which it of course wasn't, given the controvercial nature of the figure you've cited.
Also, when you're having an argument with someone, especially when you go so far as to insinuate that they're stupid, you can't rely quite so heavily on hearsay from essentially anonymous sources. What are these adcomms' names and where do they work? How can we contact them to verify this information? And how to we ensure that they aren't just giving us the standard PC boilerplate response? Or are we just supposed to take your word for it?
Also, you make the assumption that schools only give preference to people with certain characteristics if they specifically advertise it. There is no logical basis for concluding this, either. This is especially the case because it would in no way be in their best interests to advertise the fact that they make it (even slightly) easier for gays to get into their law school, if it is in fact the case that they do make it easier for them, which you have yet to rule out.. rationally, at least.