Open marriage?

We have been having a lot of problems with our sexual life (sex-life problems for 4 years now) with my husband (together married 12 years). He did not want to have sex, like ever with me and he prefers watching porn and masturbating. I asked him to cut down on it and try things together, I have gotten myself into shape, asked for therapy, anyway many things, but he has not wanted to change anything.

Then I asked what about an open marriage sexually? He did not want, I did not pressure. He then came back on his own and said sure, let's try it. Since then he has gotten massages and has been looking for us to have a threesome (with another female). 3some hasn't happened yet.

Last month I met this guy who is also in an open marriage (what are the chances?). I told this guy that for me it will only be sexual; he said fine.

I am a nice person so before I did anything, I just told my husband that I met a person I would like to explore sexually.

Now my husband has backed out of everything. No open marriage sexually, he said "if you do anything I will kill the guy" and so forth. No threesome, he does not want anything, etc, etc.

Obviously this isn't fair. And I am so turned off from my husband's behavior. I want to keep our arrangement and have sex with this other guy. Can I? Can my husband change his mind after he has done things (like massages and searching for another girl?)

(Don't mention divorce. He is a good father and husband; shares the chores, parenting, financially provides, every other aspect other than sex is great).