You're correct that homophobia probably plays a role in the male-male scenario, but what I find interesting is that even those liberals who would vote the "right" way on matters like gay marriage, would probably have a similar reaction to their openly homophobic counterparts in the male-male scenario.
But even if it were possible to remove homophobia from the scenarios, the sex of the teacher and the sex of the student would both play important roles. It seems to be assumed that the female in all of these scenarios is more fragile and less likely to be predatory. The males are assumed to stronger and more likely to be predatory. What I find remarkable is that this seems to be the case in spite of the fact that we generally recognise that teenage females are both physically and mentally more mature than males.
As for my gender, I am androgynous. I hope you find this knowledge of use
![Wink ;-)](./images/smilies/icon_wink.gif)