I can see trouble ahead! But I'm going to really throw a spanner in the works and jump to the OMs defence here.
Firstly, why is it okay for Bees to just want a shag, but it's not okay for the OM to just want a shag? He's told her he fancies her and she's beautfiful - only because he hasnt' proposed to her and also fancies his DP, it doesn't mean he isn't genuine.
Secondly, maybe the OM is a decent guy who really does 'love' Bees as deeply as one can in the early days, but can't stand the thought of being responsible for breaking up a family?
I think it must be a huge predicament for an OM if he really wants a betrothed woman who is interested in him too - particularly if it may mean ending up taking responsibility for another man's child. He may be suffering himself. When I texted 'my' OM that I was hurting he thumped back to me in capital letters 'IT'S JUST THE SAME FOR ME. IT'S NOT JUST ABOUT YOU'. If you were in OMs position, how would you act? He may be wishing he'd never met you. He may be thinking he's in really deep and is panicking, hoping that his relationship will work out with his DP - just the same as you want it to with your DH - so that he can retian a normal life and not have to deal with this crisis.
It's easy to villanise men in this situ and think they know exactly what they are doing and that they have an agenda. The truth is most men are not built that way in life. How many men think about concequences, before acting? How many men think things through on an emotional level? Men ARE predetory and we wouldn't want them to be any other way to be honest, otherwise it wouldn't be - well - very manly would it? For that we have to be a bit forgiving and understanding when they let their own desires get in the way of reason (just like all of us have let OUR own desires get in the way of reason too).
I understand he made the first move and that should be seen as a negative, however Bees says they were in a pub together for six hours. Maybe he had no intention of saying as much as he did at the start of the meeting, but found himself with a loose tongue after a few pints of beer - and ended up spilling out more than was appropriate. Over the coming days he may have worried about it and kept his distance. And then gave in when you - forgive the phrase - 'offered' yourself to him (I assume he could probably have seen it in your eyes!) when you saw him in his car.
I think you need to ask 'what exactly do I want this man to do'? If he were your puppet, how would you have him act? Do you want him to turn around and take you and your DC away from it all? Do you want to have a long-term, discreet affair? Do you want to leave DH first and then begin seeing him as a conventional relationship would start? Through being honest with yourself you will be in a position to lay down the ground rules. for example if you decide you have to have an affair then his DP may not be a problem to you. If you want a new DH, then you have to get a commitment from him now.
I told my OM that I wanted to leave DH for him and he nearly wet his pants. He was honest and said he wouldn't be happy and I have to respect him for that. It changed everything. He just wanted to be 'friends who loved each other'. I wanted him mind body and soul at the time and so knew there was nothing else I could do but to walk away.
It doesn't matter if Bees walks away now or not - she's already too far gone. But Bees, I would BEG you to sort yourself out and make up your mind about your future before taking it any physically further. I know you want him, but one shag will only make you want him more. You need to know if OM is on the same wave-length as you are so you know if/when you have to cut your losses.
Whatever happens, unless you have an affair that just 'fizzles out' in time - you are going to get very hurt I'm afraid. It'll take all of your strength to get over it - but you will get over it if you want to. I do really really feel your pain, but I was you last year at this time and was absolutely buzzing. It's been an awful twelve months, but I'd take a bit of pining over that attrocious roller-coaster ride I was on last year any day of the week.