Was it really Mirror Dax
? I thought Mirror Jadzia did not have the symbiote - so Sisko wouldn't be having sex with Dax either way (which probably would have been weird for him.)
The Dax symbiont, however, probably couldn't have cared less and may even have found the idea of Jadzia and Sisko quietly amusing...
Which reminds me, we never did get any real confirmation that symbiotes think as independent beings. Are they simply a collection of biological material that transfers memories from one Trill to another, without containing any separate identity or intelligence?