Because gay sex is icky, but Wil Wheaton's sweater is just fine.

[damned_colonial]

jmtorres puts it better than I could:

Dear John Scalzi and Wil Wheaton:

Perhaps if you didn't want people to write slash about you, you should not have named your RPF contest Wil Wheaton/John Scalzi Fan Fiction Contest to Benefit the Lupus Alliance of America.

But I guess writing unicorn-pegasus-kitten non-con scat play would be fine as long as said kitten is female? annotated_em has some thoughts on the homophobic undercurrents of the "no slash" request.