ALLOW?!? Am I missing something, or does this mean that one spouse is presuming to absolutely control the actions of another? This is not acceptable behavior, and should not be accepted- two adults in a relationship work through issues together, each side striving to understand the other's point of view and fairly present their own.
A wife that is deeply troubled, for whatever reason, by the thought of her husband watching porn, would be more than justified to bring this up in a discussion with him, explaining that she understands he may wish to do this but that she finds it incredibly hurtful, or demeaning, or even that she feels like it is infidelity on some level. But it needs to be an adult conversation, with room for compromise and discussion. He should not present porn as an alternative to physical infidelity- that is unfair and threatens the whole marriage. And she should not demand he choose between the pornography and her- ultimatums are immature, demanding, and equally threatening to a marriage.
So how should this be approached? Personally, I see no problem with pornography itself, or with my husband viewing it alone. It is not physical infidelity, it is simply a fantasy aid- and as such should pose no risk to our marriage. Pornography as a masturbatory aid is kind of a gross concept, but it relieves tension and can provide some momentary illusion of a return to freewheeling singledom that is harmless and he may find satisfactory in some way. A couple watching pornography while they are intimate is simply a choice that can heighten pleasure and is actually more "together" than each person having their own separate running fantasy during The Act.
And, as sometimes happens, if a couple intends to watch a porn movie while being intimate, and it ends up being so poorly shot and acted that instead you end up lying in bed naked laughing hysterically and making fun of the TV- well, that is just a different sort of intimacy ;) Nor harm, no foul.
Now if the issue was that he was spending money we did not have on porn, or if he got to the point where he would rather watch porn than be intimate with me, then it would be a different issue, and I would have a problem with it. So the first step would be for the wife to decide if she really has a problem with the Watching of Porn, or if it is something else- a more complete understanding of emotional issues is in everyone's best interest, and volatile subjects should ideally not be approached in an emotional way- it can escalate too quickly.
And knowing for sure the cause of the emotional trouble is invaluable- a woman troubled because her husband is watching child porn or snuff films should probably just go to the cops, but a woman offended by the thought of pornography who has never watched any herself and has no clear idea of what a pornographic movie can be... perhaps part of the "discussion" should include a little bit of a tastefully shot soft porn video. Maybe she'll change her mind ;)