A structural constraint is the level of restriction placed on your options by your social role(functionalist perspective) or by your lack of access to social, cultural, economic or political resources (conflict perspective) eg
(i) Obama has just broken the, up 'til now, structural constraint that only white men achieve the position of USA president
(ii) An over 70s applicant would have little chance of being accepted for the job of a fashion model.
(iii) non heterosexual partners have difficulty accessing rights accorded to heterosexual couples
nb it's important in using the concept of 'structural constraint' to avoid sociological determinisim. This has been explained well in the excellent article referenced below. This is the relevant extract from that article: