Big Duke, if this is with someone you already know, then there may not be a graceful way to transition into that. If you do not have a relationship like this already, then you can always couch it that you are a busy guy with no time for a realtionship, yet you desire (x,y,z here). You can say fuck buddy, you can spell some of it out if you want - monagamous, her married, only daytime - but too many conditions might be too restrictive since your pool of females wanting this type of arrangement might be small to begin with. There is nothing wrong with asking for what would suit you at this point in your life.