"it wasn't just lust because it's built over a six month period and I am not your average piece of totty."
Errrmmm ...... are you saying that because you're overweight, you can't be seen as attractive and therefore, you can't possibly be a bit of "totty" and this guy must have some sort of deeper feelings for you ??
Sorry - I'm confused here. I'm not sure if you're doing yourself down, or seeking an excuse for what happened. Or maybe both. Maybe I'm way off but you're almost coming across like it's okay for overweight people to have affairs or something !
The timescale shouldn't come into it either ..... people who've been together yonks still lust after each other, and lust is particularly relevant if there's been lots of flirting - or "foreplay" - over a prolonged period of time.
All that's beside the point anyway as I'm sure you know ..... you're married, he has a girlfriend. What's worse ... you readily admit that you're "very happily" married and "very comfortable" and whilst I would never condone an affair (having been cheated on) I can just about see why some people who are living unhappy lives might be tempted to look for happiness elsewhere. I'm sorry if this offends you but to say, basically, you have a lovely life, and still go and do something which could not only jeopardise that (but guess that'd be your lookout) but also cause great unhappiness to a man you describe as "everything you could ask for in a husband", let alone your children potentially, were this to be found out, is very very selfish.
Being overweight does not excuse you ! .... I can see that carrying a bit extra does make many people miserable, but if hubby is "everything you could ask for", it doesn't sound as if you should have any issues about needing to "prove" you're attractive to men per se, as that statement implies he's a loving man.
To be honest, I am baffled by this. No way did I agree with Nailpolish's situation, but as one similarly stressed mum, I do sympathise with the everyday problems she has. I just don't see how someone with everything you've described would give in to their daydreams like you did. (Daydreams - only - are okay)
You then go on about needing to know what he thought of it - so if it "meant something" you'll tell him it can't happen again, but if it was a mistake, you can't tell him that as you'll look silly (...... at least that's how I read that bit) ???!!!???
Look ..... if you've decided it's not going to happen again - hopefully - why on earth do you need to contact him or ask him about "it" at all ? Surely you can see that course of action might stir things up. It comes across that you do actually want to contact him and that "asking" him about "it" gives you the excuse to do so ! But that's totally unnecessary isn't it ?
FWIW, if - though you've not said anything here - there are problems within your marriage and/or life, for God's sake get them sorted out one way or another before moving onto someone else. Otherwise, you need to leave well alone ..... be professional and polite with this guy but don't return any flirty remarks, avoid being alone with him etc., he'll soon get the message you're no longer interested and I'm sure he won't have any difficulty sussing that's because you feel guilty as a married woman. There's certainly no need for a post-mortem about it.
It's a cliche but "counting your blessings" really springs to mind here (unless there's stuff you've not said) ..... so does "having your cake and eating it". There are probably 100s of women on MN who'd love to be able to say that they were both very happily married and very comfortable - you're actually very lucky to be able to say that.
I know I've gone on a bit, but you need to put this behind you for the sake of your lovely husband & children.