Relationship advice

So since my bf and I started dating over 5 years ago, I have developed a thing for BDSM. My bf isn't into it unfortunately, so I'm left with my imagination and porn. I've asked him before if he would let me play with an old fuck buddy that was into it who doesn't date guys, only girls so there would be no chance of harming our relationship but he didn't go for it :-( I would never cheat on him and want to be with him for the rest of my life but I really want to explore with BDSM. Is it fair of me to ask him for permission to explore BDSM with someone else? Or should I just drop it?Thanks for your help.