Hello Floralies,
People cheat sure but what you are saying sounds like hyper sexuality during mania or even hypomania. Firstly, don't beat yourself up with guilt. There might be a much later stage where you will want to take responsibility and deal with it when you are in a clearer, calmer state. For now, the number one prize is to 'feel better' and not worse.
I look back at crazy relationships I had or really random (but great) sex with people I wouldn't look twice at if I was stable. I became completely obsessed with this married man and no amount of advice or self talk could get through to me. It was indeed, or may be because of the state I was in, the best sex I ever had and I think about it to this day. ALL reason went out the window so I understand what you are saying. One could not say to me, but if you were 'normal' what would you do because that's impossible then.
The best tool you have got available to you at the moment is your awareness. You are well aware of this being a possible symptom of your illness. You are well aware that what you think you might be feeling (either way) may not be completely congruent to who you are. You are well aware of the value of therapy and of the support you can get in a group like this. You are also aware of the possible consequences of any actions although they might not seem like a big deal now. You are aware that might sometimes experience low self esteem and that the attention you are getting from this guy might change that in the moment.
As for telling this other man, I think that it might be a good idea. What might be stopping you is that he'll understand what's going on and might back off in an honest attempt to help you if he cares about you. He might then start to realize himself that things are not completely what they seem to be. Telling him AND asking him to support you by not sleeping with you might be the only way you can be friends with him. It can turn out to be a great friendship based on honesty. If he doesn't act on your plea then you yourself will know that he doesn't really care for your wellbeing but for sex only and that can also help you make a mind shift.
I think back at times I was 'crazy in love' now and can't understand it at all, makes me laugh, what the hell was I thinking?
So, most the advice on here says to try and wait before you do anything else and I think that's worth trying. May be jump you husband (surely he will understand that it's mania) or buy a dildo and use it all the time!
Good luck, lots of hugs,
Nosadows x