A term you could use for couples where one person is disabled and the other is not is interabled couple. Or an inter-ability relationship. Though honestly the phenomenon you are describing also happens to couples where both partners are disabled! Disabled people get infantilized and desexualized a lot, people are uncomfortable with the idea of disabled people having sexual lives, so people tend to assume they can’t or don’t have romantic partners. Thanks for hearing me out and keep up the awesome work!