i've gotten this countless times. countless. it used to bug me, then i just gave up.
i say i'm his mother. his birth mother lives in guatemala and we have not had the opportunity to meet her. i smile. i walk away if i don't know the person well.
if it is a friend, by this time he or she is stumbling over the words to apologize for the phrasing and i have an opportunity to do a bit of educating. not my job, i know, but i don't mind, so far anyway.