I used to say "No slash" all over the place in my Yuletide letters, even though I write slash all the rest of the year. I just wanted my non-canon het ships for Yuletide, dammit! But when I started to see complaints like these, and I thought about it. Were there really any unrequested het pairings that I cared to see in the background? It turned out there weren't. So my "no slash" turned into "no unrequested non-canon background pairings" and/or "I don't ship my requested characters, so please keep their relationship platonic." Which I don't think pings people negatively.