It's not really that surprising that asexuality is poorly dealt with on MN as it's such a rare thing - estimates at 1% of the population - and for anyone who's not asexual (or some variant, I suppose) sex is akin to affection or social contact or human touch or adult conversation, any of those things that you can't really say you need, it's just nice to have, but genuinely we tend to feel depressed when we don't get enough of, think about the newborn baby stage when you get crazy because you have no time for your friends any more - it's really difficult for people who experience that aspect of sex (which is 99% of people!) to understand what it would be like not to experience that at all. And actually to expect somebody who does experience sex in that way to have a lifelong relationship, which of course a marriage is supposed to be, without sex is unfair on them too, just as it's unfair to expect a partner who finds sex to be a nuisance to regularly engage in it just to keep their partner happy - that's just not good for anybody. It doesn't work because the one who wants/needs sex wants/needs it with a partner who is engaged and enthusiastic, not just somebody unhappily going through the motions, and for the partner who doesn't enjoy sex, going through the motions can also be pretty damaging and crappy of an experience in itself.
Also, perhaps the idea of extramarital sex does work for some asexual-sexual couples but I have huge doubts about this, to me it reads like the kind of clumsy, top-down solution somebody comes up with when they've no experience of the situation from any side, kind of like "If your partner feels left out when you're breastfeeding, you can always express milk for him to feed!" Perhaps it works between a couple where one is asexual and the other sees sex as a purely physical thing that they can separate emotionally from the relationship, but I'd guess this kind of combination is rare.
Gemini's sentiment is lovely but unfortunately exactly how people end up in unhappy marriages, because we are prone to tell each other and ourselves fairytales about how our relationship actually is and what we can handle when we're faced with shitty situations, especially when the wedding is imminent. It feels like such a huge cost to admit that there are issues at that point that it's too dangerous and scary to actually look inside ourselves and see what the reality is.
Obviously, you should talk to him but you can't just accept blindly that he's happy and okay with something if he's saying these things with one hand while making offhand comments that give away his true thoughts with the other. You've got to have these hard conversations together, and truly, life is hard enough on marriages at the best of times, without one (or both) of you feeling like you've made a significant sacrifice to be there.