I was going to post something along the lines of timeisnotaline.
Rather than the sneaky option of getting you to agree to an open marriage so he can actually go and get his jollies elsewhere, I read this as a desperate man, who fancies you, loves you and doesn't wan't to lose you, so is frantically coming up with options to "keep" you, whilst also actually having sex.
I couldn't and wouldn't, be able to to keep up a relationship that had been sexless for that long, even if I loved them with every fibre of my beinf.
I suspect he has been unhappy for quite some time, and despite the cynical thinking of some PP's, I actually think he has not said or done anything coercive up till now because he loves and respects op and wanted to give her time. (raging feminist btw, not an excuser of men at all!)
No one should be coerced into sex if they don't want it, and no one, ever, should have sex with someonejust to keep a relationship together.
But equally it is totally out of order to expect anyone, male or female, to live life without sex happily, if they are aching for it.
But then I see that you have had an affair...so you know the answer.
Stop stringing the poor bloke along just to keep your "happy" (yeah right!) home together for your adult, and non resident "kids".
They are grown ups and will understand.
If you really did have a boyfriend then you are the worst kind of money grabbing, lifestyle preserving hypocrite and I hope your husband ditches you.