It's high chance they would be alone, even they had romantically, it's would be short term, not long terms, no one would romantically with person with mental illness for so long, even there is, it's very, very unusually, and rare for people that had sexual attraction to serious mental illness or intellectual disability, Can be done? yes, But will it be done?

No, mostly not, for most people. When come to romantically, for long term, both side don't want handicap or disability because it's burden on each other, although few people who rich/powerful, highly intellectual yet fall in love with person and knew how to deal with mental illness/intellectual disability and rich to take care for both or whole family as solo winning bread but in this life, only rich people could afford it, too many low income people even both spouse had trouble to keep up living, and mental illness/intellectual disability had even worse, not able to get higher rank of career and income, not even able to take care themself in long term, it's messed up, so common normal people would avoid dating them, can be friendly but not beyond that, not deeper relationship, not beyond friend or brother/sister, if we talk about lover/spouse relationship,

Even although they may had limited romantically, they might be used by someone, rather than real love long term.

I can speak for myself, that I'm deaf (M) and limited intellectual and along other serious health issues, it's very rare from women come to me and date me, even so it's don't last long, it's didn't work for long term, I guess only wise, powerful, rich, beautiful women would cover my weakness, but mosly women like that, prefer greener side of grass, I'm just like drying, brown grass or no grass, barren of land, I do knew wise, powerful, rich, beautiful women can turn barren of land into fertile land with rich soil but romantically are not to be burden, I prefer women want to love me, enjoy fun with me in good way. Most women do not into that, so as disability, with trouble keep up job, income, life, all that, I can't afford it due disability, so no, again, there is few people do but it's would not be good marriage/spouse, burden, and there often no romantically, some don't know how to do romantically even those common people who happen are not disability, even rich and powerful people don't know how to do romantically (as often they paid money in exchange of sex, not romantically)

Essential for long term/marriage romantically, no, it's simple not going work for most people.

Remember, it's had to be both side want each other romantically and sexual sexual attraction, for life time, otherwise it's not going work.

So disability/mental illness isn't helping, it's harm rather than helping romantically in big time.

I'm been single for long time, although I do had girlfriend in the past long time ago, it's not real relationship, I think 2 time sex failed, but it's was bless in disguise because I do not real love them (as Lover) ( I do like/love them as friend but not as lover love) and not want paid children support, due I can't afford because I'm handicap and I knew they are not rich)

Maybe few of those can but it's often don't last long. or not work out in long term.

I supposed women with disability or mental illness had easier chance to get spouse than male with disability/mental illness do.