I think all you can say is "You're an adult; I certainly can't control what you decide to do or not do while we're separated. Personally, I don't think it's right for me to date until we're divorced, so that's the route I'll be taking," or something similar.
Or, if you DO want to date, something like suggested above.
My wife originally INSISTED on us dating others while separated; in fact, it was a big reason why she wanted to be separated. After initially fighting it for several weeks, I finally reversed course, and told her while there wasn't anyone specific, I was looking forward to trying it out, to see who might be out there for me. Well, that COMPLETELY changed her mind, and on the night I went out on my very first "date" (just met a former co-worker for drinks), my wife called me in tears, and wanted to "talk." After some heartfelt conversations about it, we agreed to remain separated, but to date only each other exclusively, and things have been going great ever since.
