In the US your partner’s income would still count for welfare benefits and the like, because those things go by “household income”, and in such cases children’s incomes and roommate incomes would also be considered.
So if you were a single person with a child who worked after-school jobs and a roommate, and applied for food stamps or other assistance, all three incomes would be considered (and possibly reduce your benefits) but no one would argue that you were not single.
Possibly not “treated as single” but legally single nonetheless.
To my knowledge, my housemate’s income was never considered for anything at all when a friend of mine and I lived together for about three year’s or so. I never applied for social assistance of any kind though.
That’s why then. Social assistance…things like food stamps, welfare, cash assistance, reduced lunches (for school lunches) etc. those are the things that use household income and in which an unmarried partner’s (or roommate’s) income would be considered. It does not make them consider you “not single” though.
As far as I know only social assistance programs and joint ventures (loans or credit in both of your names) would consider a partner/housemate/etc.'s income when one is legally single (not married).
So that was kind of my point that even though those programs consider who you live with, they do not consider you legally “not single” they just use household income while making no comment on the relationships of the household.
If you’re asking about marital status, I am single. If you’re asking about relationship status, I am not single.
The word “single” can mean either “unmarried” or “unattached,” depending on the context, though the latter meaning seems to be more recent. But it’s just one of those words that can have more than one meaning. (For example, my brothers and I are our parents’ children, but we are not children—we’re adults.)
To throw a curve ball in the works here, I’m in a polyamorous marriage. My wife and I are in a “committed” long term relationship but we are not monogamous. We are “available”, but obviously not single.
(The scare quotes are for other people who might not agree that we’re committed and/or available. We certainly are, though it is probably a non-standard usage of the terms.)