No it's never okay to cheat. Deception is never a good idea in a relationship.
Is it okay to have sex outside the marriage, if he agrees to it, yes.
Your problem is not the lack of sex, it's not that he's disabled, it's that he's totally unwilling to discuss something that is upsetting to you, and unwilling to even attempt to get help for it. If he tried everything he could and still was unable to have PIV sex and you left him, it wouldn't be because of the lack of PIV sex, it would be because of incompatibility.
That said, you're asking him to have sex with you when he really doesn't want to, and sexual pressure is awful. You're telling him that the lack of sex is a deal breaker. If this were a post about a man and his disabled wife, I think many of these responses would be different. He doesn't have to have sex with you if he doesn't want to because he has bodily autonomy.
To be honest I don't think sex with another person is a need. I think intimacy is. I think sexual release is. Emotional connection. It seems like many of these things are missing and that's sad because I wouldn't feel loved at all.
I do hope things get better OP, but I don't think going outside the marriage is the answer, even with permission. You will probably end up emotionally drifting away from your marriage and end up leaving anyway. So do some serious thinking about what you really want and if your H isn't stepping up to the plate, it might be time to leave.