Probably best to be honest, it will only hurt more if you carried on "sleeping together casually" and then if she ended up sleeping with somone else (not saying she would but IF). It would just end up burning you to the max!
Nothing wrong with thinking about the future, but you cant realy focus on it 24/7, you've got to focus on the present too! Just take every day as it comes, you know the whole step by step, "cross that bridge when you get to it" scenario! But yeh, so long as your happy in yourself at the same time. You are right though, there is NO point falling out over it, (me and my ex dont talk anymore, it all ended on bad terms because he was just -to put it bluntly- a complete ******** who didnt treat me "well"). Its probably better to be friends and know that you can be there for each other emotionally as well!
But then again you never know, i guess everything happens for a reason, you just got to see what the future holds (you never know, you might meet somone who you fall completely head over heels in love with and spend the rest of your life with them)!