Erm...well, let's first say feelings are hardly a thing you can control. So, there's no point trying to fully rationalise how you got attached to that person. Or, for that matter, flog yourself over it. That's said, do you think you find in that person a refuge from your own guilt towards your partner? I mean, since you cannot have sex with him, you probably feel burdened by that and perhaps worried he might look for somebody else. You might be jumping the gun, you're probably preempting the worst.
This is only my assumption, and I'm not saying it's necessarily true.
Then, there's that fact that your husband isn't bothered about the lack of sex. Erm, I'm quite sure whether this is how he actually feels or he simply doesn't want to hurt you? At the risk of putting suspicious ideas into your head, I personally, as a man, don't have full trust that a heterosexual man in a marriage would go on for years without sex, can he? Could he be asexual? Some men are, and some of us develop that temporarily due to circumstances related to stress, depression, shame...etc. Or, as you say, his emotional needs are similar to those between a mother and her child. Could this be a reason he's at peace with the idea of a sexless marriage? Is he looking for a mother's figure rather than a wife? Maybe you could gauge his family life, especially his relationship with his mother. This will probably give you some insight into his psychology...
Also, I understand you say you don't plan on acting on your feelings towards the other man. But, again, those are feelings...and there's only one way they'll go if you continue to see each other. At some point, it's possible that you'll become overpowered by your emotions towards the other man and end up in an affair with him. An emotional one to begin with. But what's next? Does the other man know you have an issue with sex? Will he be OK with that?
I'm sorry if I come across a bit harsh. I just identify with much of what you said, and I know it's really hard. I totally sympathise. But I try to give you my insight since I've been there at some point. Perhaps you can pick up some truth in what I say and build on it.
The question you should ask yourself, at the end of the day, is what YOU really want? I'm not suggesting a Machiavellian approach where you justify your own needs at the expense of your husband's, but something that would make you happy while keeping some fairness and balance.