Is it ever okay to have an affair

Yes, at least there would be no emotional involvement. Also if he stopped watching porn it might help with his 'needs'

An affair ? No. If he must then would a prostitute be the answer?

An affair is a betrayal of trust. Just not on under any circumstance.

quite right HildaW my thoughts as well.

Being a simple soul I've always thought that on becoming a couple...whether through marriage or just mutually agreeing to be someone's 'one and only' one is sort of saying 'we have something special and do not want anyone else'. Thus by having an affair one is stepping outside this prior arrangement and betraying the other partner.
What is left is a mess and can, in some circumstances, be sorted out or allowed to pass but, the original relationship is now very different.
I think what I am saying is that if you want your original partner to be there for you....you pretty much have to play by the rules you originally agreed to.
If, as seems to be the case here, one partner has pretty much failed to live by the arrangement....i.e. not emotionally and physically supporting the other....then its up to them to re-negotiate or call it all of.

... only if you could be bothered! (in response to the OP question).

Well, she has said. 'No sex, that's the way I am'. I think under the circumstances he can say, and should say it not just sneak off and do it. 'I need sex that's they way I am. I will seek it elsewhere'

If I was the wife , I'd be more upset about him watching porn than him having an affair, but it would depend what the affair entailed.

Only thing is we might find desire is rekindled with a different partner, and he, the new one, remain uninterested in sex.

Maybe he has just realised his own mortality, and doesn't want to end his days knowing that he is never going to have sex ever again.

I have a good friend for whom sex is never going to be possible again, for medical reasons, and she is dreadfully worried about it and the effect it is having on her marriage.

If only life could be simpler, we could all pair up afresh with partners with the same desires as us.

It's a difficult one to answer isn't it.

Whilst she has the right to refuse sex if she's not interested, does she have the right to insist that he abstains as well?

If it has gone on for 25 years, why is it now a problem? Has he spoken to her to find out what turned her off sex? Is he the type of man that if you so much as hug him, he's sees it as a green light for sex (I know many women find that off-putting)?

I don't know if an affair would be the answer - it would depend on if he is willing to risk the end of his marriage.

What an awful dilemma to be in.

It seems that this couple have been having problems for 25 years. The wife not only rejects the husband sexually, but angrily rebuffs him. Having an affair won't mend their marriage, but just drag someone else into their problems. Many couples find their own way through the difficulties of waning sexual interest, but this is a longstanding problem for which I would think they need professional help.