Something like: "I know that the vast majority of people aren't successful in MLMs but I also realize that those that put in the time and lots of effort and contact a lot of people can be successful. I just look at my life right now and I'm in no way in a place to be able to do that. This isn't for me"

If that doesn't work my next line would be:"I just hate the thought that I'd have to be the guy that's constantly bugging my friends and family and that won't take no for an answer" (I think they'd realize they are now that guy)