Let me see if I've got this right. You and hubs were both raised in strict Christian households. In your infatuated dating or early married time, you admitted you were bi and would prefer an open marriage where you could have both a male partner (him) and a female partner. You and fiance/husband went to "parties" where you "played" (had sex with) other adults. Mostly you did this, but husband did it some and, according to you, he enjoyed it.
So, he knew you were bi and not mono. You knew he was asexual/greysexual. But while he is a little willing to have sex, (and things looked more promising early on) it's not nearly enough for you now. And he's totally embarrassed to talk about sex or having an open relationship. He shuts you down EVERY TIME.
Finally, after about 15 years, while you've been doing therapy individually for a while, he has also agreed to get individual therapy. But he won't do couple's counseling. I guess that would still be too embarrassing.
I know of many people who were raised in high-demand Christian religions/cults. They get such harsh brainwashing about no premarital sex that even when they do get married, they can hardly have sex. If they didn't feel pressured, as good Christians, to breed, they'd probably avoid sex even more. The abuse around the subject as a kid/teen can make this a really high hurdle to overcome.
You can't hint around about poly. You can't just bring home a likely woman and hope she'll be attracted to both of you, and husband will want to bang her as much as you do. Triads don't work that way. (Triads are way too hard to do, especially for newbies, anyway. So get that idea out of your head.)
Husband won't talk about polyamory. But I am getting the feeling you don't understand it very well, either. Please see the Resource thread that is a sticky at the top of this page. There are books, websites, articles and podcasts listed there that are incredibly informative. You can learn more about how polyamory works as husband starts this therapy.
And keep posting here! We are your people. We understand. Even Seasoned means well, although her delivery can be a bit harsh sometimes...
You're not the first married person to have a mismatched libido to your spouse. You're not the first bi person to post here. You can search terms here to find older threads on these subjects and many others. Enjoy your research!