I have to agree with nut.. been there done that..
My ex was supportie of my other relationship but when he got into one..ok there were some fireworks. If separated i don't know if its fair to the person you are seeing if you get serious.. At any point you have the option of going back to your significant other and leaving said person with a broken heart.

For some ok not a problem..for others there is nothing like a broken heart and trying to pick up the piences..

Ok i am getting off soapbox now.