@deathmailrock: Remarkably well said. When it comes to ecchi/H content, it is all about tastes and fetishes--and authors know well enough
not to mix diametrically opposed ones, or else they'll end up cutting their audience in half or worse. On top of that heterosexual-oriented content walks a relatively narrow path, unlike the alternatives: if you have a heterosexual MC with a harem of girls around him, and he decides he wants to add one guy to the group, you don't have a heterosexual MC or story anymore, even if the guy leaves the group.
On a side note, this reminds me of something I've seen for many, many years, now: the LGBT mafia has had this fetish for "turning" heterosexuals. (Makes sense, really--it's the only way they can propagate.) They like(d) to see them falling for traps, they like(d) to see them "slip", they like(d) to see them branded so in one way or another--they especially like(d) to see the aforementioned in their critics/opponents. It stands to reason that you would have said mafia and its supporters approving of a mangaka throwing a guy into an all-girl harem, or criticizing a mangaka who does not.