I'm surprised that your reason for not having sex for 3 weeks is that you're waiting to have a coil fitted, do you reckon that condoms are too risky? You could also have non-penetrative sex play, like rolling around naked and using your hands on each other.
It's one thing if for whatever reason you don't fancy having sex at the moment, but it seems a bit silly to refrain from all sexual activity while you wait for the coil, if in the meantime both you and him are horny for each other.
I'm in agreement with Walkingdeadfangirl that what he's looking at wouldn't be considered as 'porn' by most people in this day and age. And I think she's right, his Christian faith is no longer the same as yours. He doesn't want to pray, and he doesn't consider looking at naughty pictures to be sinful in the way that you do.
A lot of hardcore porngraphy videos are disgusting and promote abusive attitudes towards women, so I could understand you being upset with your husband for secretly watching porn. However I did a search for the subreddit you mentioned - it turns out it is actually called 'SexyGirlsInBoots' - and as you say, there's nothing hardcore, it's just suggestive photos, women in tight outfits or with mild nudity. They are 'naughty' pictures, but there's naughtier things on page 3 of The Sun, or films or TV shows that are on after 9pm. Or a women's fashion magazine!
You could argue that a good Christian shouldn't be looking at any of those things, but that kind of imagery is considered pretty commonplace to most non-religious people nowadays, and even if it is somewhat sinful, it is very mild compared to the sorts of things he'd be looking at if he were actually addicted to porn.
I think the main point is that he's looking up this stuff as a visual aid for masturbation. Both men and women like to look at sexy things when masturbating, but if he wasn't looking at pornography, he'd simply be imagining the sexy imagery in his head instead.
So I guess the question is, do you have a problem with him masturbating? Do you consider masturbating to be against your faith?
In many marriages, there wouldn't be considered anything unusual about either you or him masturbating if for whatever reason you hadn't had sex for three weeks. I know that some Christians (and people of other faiths) consider masturbation to be wrong, but I'm a Christian, and I've never believed that.
If your husband preferred to masturbate (and look at naughty pictures) rather than have sex with you, THAT would be awful, and is a problem I've seen many times on mumsnet. But it seems to me like he's just horny and frustrated that he can't make love to the woman he loves, and he's relieving that frustration.
My fiance and I have been in an absolutely devoted relationship for 4 years. There was a time when my partner and I ended up in a bit of a long-distance relationship, we had jobs in different cities, and we could only be with each other at the weekend. We missed each other loads, and sometimes during the week we would both masturbate - sometimes we'd do it 'together' while having sexy chats with each other on the phone, but sometimes we'd just do it on our own. And when on our own, sometimes we'd just use imagination, and sometimes we'd look at porn, or read an erotic story.
It didn't mean that either my partner or myself had a 'wandering eye' and was lusting for someone else - we were lusting for each other, and we wanted to relieve those feelings rather than being frustrated all week.
I think that your husband does still have a strong sense of morality, and shame about what he is doing, and that's why he's look up mild naughty pictures of 'SexyGirlsInBoots' rather than explicit pornographic videos.
However, he is wrong to expect you to not call him out on it, simply because you're supposed to have 'forgiven' him. That's not how forgiveness works. If he doesn't think that what he's doing is wrong, then he should argue the case for that, rather than throw your previous forgiveness in your face.