Oh I do grin and eye-roll a bit about how many excuses MNers will find for a woman who's fallen in lust with a married man....loneliness, boredom, lacking in confidence. Have we not had that perennial favourite yet... low self esteem ?
Hats off to you OP for having none of this. It actually sounds very banal and straightforward. You're very choosy about who you have the hots for and you've met a nice bloke who you get on well with and who you'd like to shag. As that doesn't happen very often for you because you're discerning in your tastes, this has blown your socks off a bit.
As for him, it's probably vaguely similar. He's married not dead and he's probably as happy as er...Larry (not Grylls 
) at home but this new friendship has put a spring in his step and he thinks it's all quite safe because he won't make any overt moves to step things up a gear. There might be a bit of mentionitis about you at home and possibly his wife's ears have pricked up a little about this new colleague of his.
We could all be terribly pious and say 'what a prick' for not mentioning his wife during the introductions, but society is terribly double-speaking about this issue. All of us right-on sorts believe that men and women can be mates and that friendships at work are A Jolly Good Thing, but what we aren't so hot on is admitting that sometimes they cross over the line and that folk don't always realise when that line has been crossed until it's too late. Up till then they are too busy parroting the 'just a colleague I hit it off with' line to themselves and any mildly concerned partners.
So here's the deal.
Back off.
Under no circumstances tell him how you're feeling. Because if you do, only two responses will come back.
- He will say 'me too'
- He will affect surprise and tell you that he is terribly flattered but he is married and doesn't want anything more than a work friendship.
If you go all moist at 1) then you're hurtling towards an affair and only the willpower of the Superhuman would stop you.
And if it's 2) you'll be horribly embarrassed, going in to work will be shit for a while and at some point you might even blame him a bit for 'leading you on'.
There is of course a third option which will be the 'hedging his bets' one, involving him giving some weak 'chase me' response of how much he fancies you, wishes that things were different, thinks about you a lot, is a bit fed up at home....but, he's married (sigh) which would keep the flirtation going, requires you to chase him, wear him down and would eventually end up at the same point as Option 1).
Look at the positives.
You feel something for a bloke. It's whetted your appetite and you know everything's in full working order. Just treat it as a little 'awakener' and get out there again meeting new single blokes. Don't stop being discerning but remember, if you've clicked with this bloke, you will click with others.
He certainly won't be the only one. Think about the flawed logic of meeting the greatest bloke of all time in one workplace in one town. Meanwhile there are other great blokes in other towns, workplaces, hobby groups and lots of them will be available.
It really is no big deal to want a shag if you haven't had one for a while and it's as plain as day that seeing as he's probably the only half viable bloke within the vicinity and you see him every day, all this longing has focused on him. It doesn't mean you're lonely, depressed, under-confident or will be forever attracted to Messrs. Unavailable. And he's no Mr. Wonderful; just an everyday niceish bloke you happen to work with. In short, you're probably just a bit sexually frustrated that's all.
Don't make the mistake of turning this into something it's not. Lusting after someone doesn't have to become an obsessive infatuation or worst still, true lurve unless you let it. Own your sexual appetite and embrace it. It's really nothing to be ashamed of.