Do I have to tell my condo association if I’m married?

First of all, you say "one of the units" is asking. If it's just another resident asking for his or her own curiousity, they are free to ask--anyone, as a private citizen, may ask another person anything they like. You are free to answer, ignore them, etc.

If it's the actual association, they may also ask...but they can't compel you to answer unless there is something in the HOA agreement or a similar document which requires you to respond, such as some term or condition there which obligates residents to identify any one living with them. (There are valid reasons for such a requirement, by the way: for example, a spouse has certain rights over property which a non-spouse does not, so there may be a legitimate reason for them to have that information.) In the absence of some requirement, such as in a HOA agreement, for you to answer, they can ask and you can ignore them.

As for how a "unit owner" is defined--if there's any definition in any agreements govering the condo, those definition(s) will control. Otherwise, as a general matter, an "owner" has an ownership interest--he or she literally owns at least part of the unit. It's basically the commonsense, everyday definition of owner, unless there is some specific definition to the contrary.