The earliest use I can find of "frau" in Datalounge is in 2006, but its appearance preceded that.
Etymologically, it is an abbreviation in form of "Hausfrau," which as you know, OP, literally means "house mrs." or "housewife" in German.
Its first appearance in English was in 1798. (Honestly.)
However, in its German form it took on a sense of a stay-at-home woman raising her children and being clueless and ignorantly opinionated, usually in the direction of "You ought to....", "Can't you be nice," and "That's not how we do things."
Today, this derogatory sense is understood both in Germany and the US. Its current negative meaning took firm hold with the Women's Liberation Movement, often used by women against women who were seen as holding their sex back. This meaning was readily adopted by gay men (who likely were using it prior to Women's Lib, trendsetters for female objectification, good and bad, historically).
At the DL "Hausfrau" was simplified to "frau" (usually with no capitalization, which is appropriate in German). The DL sense combines all the negative connotations, with the added component of homophobic presumption, even when superficial pro-gay intentions are claimed.
The appearance at the DL of straight women of various backgrounds and sensibilities (or the assumption that opinions or stances unliked by a poster inevitably were posted by a woman) triggered (YES, TRIGGERED) a strong response that still is visible today. Also, the frequency of usage here has, at times, lessened the intensity of meaning when the word is used.
An additional comment that needs to be made concerns the use of homophobic postings as a (pretended) righteous reaction to "frau." You can see it on this thread. A particularly vile example was the "Sexism is death"/"Sexism is killing us" troll who posted the same pleading post every time s/he saw the word. No it is more usual that such creatures respond to "frau" with "fag" or, sometimes, "AIDS-riddled fags." The false equivalency is obvious, and reflects the lowness, trollery and hate-speech unwellness of a few over-posting offenders here.
The sort that doesn't know what "conjugation" means but knows that it is something ugly in its own mirror.
Glad to help, OP!