Wow, that is a tough one. You sound like a person with a normal sex drive partnered with an asexual man.
Or perhaps he's gay?
He can't or won't fulfill your need for a healthy sex life, but objects to you going elsewhere for sex.
Leaving desire for others out of the equation... that's a tough situation all on its own. I am glad you sought treatment for his sexual dysfunction. If it hasn't improved with 2 years of sex therapy, maybe he just shouldn't be married to you. He might be more content with another asexual partner, and you might be more content with one or several other lovers who are on your wavelength.
I hate when people on the internet just go, "Dump him." We know so little about your situation, but it sure sounds like you need to find a new way of living and loving. Life is short, too short to live in misery.
Love withers under constraint; its very essence is liberty. It is compatible neither with envy, jealousy or fear. It is there most pure, perfect and unlimited when its votaries live in confidence, equality and unreserve. -- Shelley
There's no lying in polyamory!
I'm a 58 year old woman with 2 partners:
miss pixi, my live-in gf, 36 (together since Jan '09)
Ginger, bf, 61, married, lives nearby (together since Jan '12)