By marrying before the birth they ensured you were legitimate, which may suggest that they intended to try and keep you, but it isn't unheard of for a married couple to have a baby adopted. It is much less common than for single mothers but perhaps there were financial or other family reasons why that decision had to be taken.
One possibility may be that the man she married wasn't the biological father, and the adoption was a condition of the marriage going ahead, if he didn't want to raise another man's child.
Your adoption records, which you can access (if you haven't already) would usually have some information on the family circumstances and the reasons why adoption was being sought.