I am definitely not asexual, love me a bit of penis in-out-in-out when I'm in the mood, but I don't find the concept of asexuality hard to understand at all. Don't know why others are being so weird about it. As someone previously mentioned, literally everyone has experienced what it is like to be asexual - as prepubertal children. So it's not a difficult thing to imagine. It doesn't have to be imagined it can be remembered.
In other respects it's easy to understand if you just think about what your own sexuality isn't. If you for example are a man with no sexual interest in other men, the thought that you would allow another man to engage in sex with you out of duty, loyalty, a debt owed etc is abhorrent. No matter how great he was, how much he'd done for you, how much you liked or loved him, or how sexually unsatisfied he was. I really feel like noone would view "have sex with him anyway" as an option. They'd rightly view it as a violation. It's clearly the same for OP and her husband. His choices are to remain in the marriage without sex, pursue sex elsewhere or leave the marriage. No decent person willingly engages in a sexual encounter they know the other person finds violating. It simply isn't an option.
Its good that a label exists because it can help people to understand themselves, validate them in not choosing not to engage in unpleasant, violating or traumatic sexual experiences, and helps asexual people to find and date each other. It really is completely analogous with gayness in that respect.
No doubt some people experience fluidity in their asexuality (just as with other sexual orientations) whereas some will feel consistently asexual for life. Again, no one would say don't use the label gay in case you are ruling out potential heterosexual feelings in future - labels can be changed!
Sorry people are being so rude OP. Sorry you have had so many unpleasant experiences on your journey to understand your sexuality.
It's your husband I'm actually more curious about. I do wonder if he has a lower sexual drive than average. Or sex isn't that important to him relative to others. In my marriage, which is sexual, it would damage our connection to one another maybe if one of us just totally withdrew from sex but that doesn't seem to have been the case in your marriage. I guess it might be hard for you to gauge your husband's sexual drive because it's always going to have been infinitely greater than your own. Do you feel he is less sexual than other men you dated previously? Might not have been part of his appeal for you? Just curious. Obviously an extremely personal question no need to answer.
I would also remind you that no doubt your husband has had many other lovely things as a result of your marriage, not least of all your two children. He clearly prefers to remain in the marriage (and to stay with you and marry you despite what sound like, very little sex throughout your relationship). Just having the thought that it might not be "heartbreaking" as such. Does he really view it that way? Or is he actually a pretty happy chap?