First of all, a plea to folks to use HTML or BBCode for long URLs, rather than pasting the long URL directly into your post. The HTML code looks like this:
<a href=http://duskpeterson.com>My Website</a>
The BBCode looks like this:
My Website
And then uncheck the appropriate box below where you pasted the text. The result ends up looking like this, with a link attached:
My Website
The reason I'm asking folks to do this is that, every time a long URL appears in a post, the text lines stretch to infinity for anyone reading in large print, as I do. I have to go down to standard-sized print to read the posts, which is difficult for me to do at this time of year.
"If your audience is het women who like to read m/m, why would they be bugged by a hot het scene?"
I agree with the others that heterophobia can play a role, but I can think of other reasons:
1) Genre preferences. If I like mysteries, it doesn't necessarily follow that I like thrillers. If I like gay romance, it doesn't necessarily follow that I like heterosexual romance. The elements that draw me to gay romance - perhaps it's male bonding - may not be present in the het story.
2) Erotic preferences. We can't assume that what people find sexy in real life they'll find sexy in writing. I've met lesbians who are turned on by m/m slash. Similarly, I've met heterosexual slashers who are turned on by m/m stories but not by m/f stories.
3) As has been pointed out, some people find it more traumatic to read het because of the long history of sexist het stories. It's nice when a story breaks beyond this, but some people aren't willing to take the chance of reading another story that might kick them in the guts.
4) Some people simply have personal reasons for not wanting to read het - heterosexual sexual abuse in their past, for example.
Now, the folks who say, "Eww, girl parts! Cooties!" are just plain silly, of course. But those are often the same people who are saying, "Eww, muscles instead of androgyny! Cooties!" So I don't really worry much about their opinions; they're going by fashion rather than giving any serious thought to what they read.
I don't write bisexual stories per se, but most of the male characters in my "gay" series are actually bisexual. I've added heterosexual subplots into my stories and gotten no negative reaction - but I don't write explicit het, so that could make a difference.
"I don't know much about slash history except that it started in the mid-60s and was VERY underground."
The 1970s, actually; as I recall, there was one 1960s story that wasn't published till the 1970s. And yes, it was very much underground for years.
"if a writer consistently makes a recognisably seme/uke dynamic between, oh , say-- Jack Sparrow and Commodore Norrington to name one of my faves-- she'd get called on it after a while."
Perhaps so, but that sort of thing is still quite popular in slash. It's been commented on quite a lot by scholars writing about slash. It may not be as obvious to most slashers as it is to me, since I read a lot of male-written gay leather porn where the dominant/submissive dynamic occurs, but the submissive won't be feminine in his submission.