How many of these never-married men are in a relationship? Unmarried != single. Stats on the number of singles who have never been in a relationship would be far more valuable. Relationships, sexual or otherwise, are not subject to paperwork. The days of relationships, sex, or even reproduction being tied to marriage are long gone.
In most cases it just means “not in a relationship” and doesn’t tell you about marital status. But if you’re in specifically asking about marriage, it can mean unwed (and I’m sure it gets even trickier for poly relationships).
If a flirty stranger approaches and asks “are you single”, you’d say no “no, I’m in a relationship”
If you’re filling out a form at the doctor office and it asks “marital status”, you’d put “single”.