What is your take on live on BF (M58) no longer interested in sex with me- his GF(F50)

A little background info.  We have been together 4 years.  Always had amazing sex.  Then suddenly... it just stopped.  It's been months.  I have asked what is wrong, and I get the answer that his libido is just very very low.  He's not interested in anyone, no porn, no fantasies, nothing.  I explained that I missed having sex. I miss that closeness.  I've tried doing all different things... to no avail.  He said he would try to "start fantasizing about me" and have sex. And it was awful.  I have learned that pity sex is the worst. The thing is... I know he's lying about his libido.  I have come home when he was home and have heard him pleasuring himself.  I know he's watching porn.  He still gets erections.  He claims he still finds me attractive.  But his actions say otherwise.  I am at a loss.  He has been married before.  He and his last wife ended up opening up their marriage due to loss of interest in sex on her part. When we met he was in multiple relationships.  I am not into that and stated as such. I'm beginning to think he is just bored with this monogamous relationship and is going elsewhere.  Even though he claims otherwise.  I am nearing the end of my rope.  I feel like this is a foolish reason to end a relationship. But I can't reconcile being in a sexless relationship either.  I'm too old for this crap.  Being lonely while being with someone is the worst feeling.  I'd rather have a gang of cats and a kick ass toy.  

I would appreciate any advice, suggestions, etc.