Whether it's been 12 months, or 21 months, without sex, this is FAR too long.
How utterly frustrating, to lie naked next to your Partner, for a year or more, with no intimacy whatsoever. No wonder he is snappy. I would be too. Yes, yes, he's not covering himself in glory, and he most likely knows this, but for fucks sake, what gives? He's not an emotionless robot....he is bound to have strong feelings about this. Every time he sees a sex scene on TV, or something that turns him on...he knows that there is NO CHANCE of any intimacy for him. And he's only young. I think the threats about looking at other women, and shock tactics, to try to spur the Op into some kind of action.
It's all very well, PP saying things like "you don't owe anyone sex"....no, you don't, however, a person has no right to deny their Partner sex, and then demand that they don't seek it elsewhere (eventually). You can't force someone to be celibate, ffs.
I sometimes think, that there are a hell of a lot of people with low/non existent libido's, that just have no comprehension of how a person with a high sex drive feels, when they are denied sex by their Partner. It is not tenable, in the long term.
I would go so far as to say, that if the husband in this scenario did eventually cheat, that it's difficult to label it as cheating, as he's not having sex with 2 people at once. His wife is essentially his room mate at this point. There's no sex, no intimacy, no bonding....they are just co-parenting, which is not enough. Well, it's not enough for the person who still wants sex.
Honestly, I think Op has to try to get intimate here, and hopefully this will reignite something.
If you keep ignoring this, he will eventually fall in to bed with someone else. He's a man, not the Pope. And I'd say the same for a woman who was having intimacy withheld.