How is your relationship apart from that? Do you love each other? Do you have fun? If you spend all your time bickering and nagging at each other, then I can see why things might be dead in the bedroom.
When he says he's 'not interested', why is that? Would he like to be interested? Because there's a big difference in 'I've lost my sex drive and I'd like to get it back' and 'I don't really enjoy sex and would happily never have it again'. If it's the former, he can go to the doctor and get some hormone checks done.
Also, how old are you both? There's a big difference between a man losing interest at 70 and a man losing interest at 35.
If you want to have sex with other people, and you think that's a conversation that won't devastate your husband, of course you can talk to him.
But given that you apparently like the idea of a 'friend with benefits' - someone you really like, trust and sexually compatible with - rather than meeting people online or whatever, I suspect that what you actually want is to leave your husband for someone else. You haven't said anything about your husband that indicates why you want to stay with him and have sex with someone else on the side, rather than simply leaving him to find someone else (or start a relationship with someone you've already found).