Obviously, you are facing a cultural obstacle. The problem seems not to be directly connected with your marriage and the 18-month gap alone.
Not being in Pakistan and having never visited, I can only comment from secondary information. Marketing requires a huge amount of traveling, exposure, and interaction with outsiders. From what I know of Pakistan, that is not suitable for women, especially married ones (culturally, I mean). While Karachi is probably as cosmopolitan as Mumbai, the cultural mindset of employers (or more likely the recruiters) seeps in. So, given a choice of other male candidates available, you are not likely to be selected for a sales job.
However, there will be specific jobs even in marketing that require women. For example, in the marketing of a chain of beauty salons or women's health and hygiene products. You need to identify them and try for those positions. Actually, you should have taken these factors into account when choosing your MBA specialization.
Companies prefer to take ex-army candidates for admin positions mainly because of their discipline and organizational skills. In India, army candidates are preferred for security and admin, but in admin, they would generally be in senior management posts. Other posts are filled by normal employees.
Anyway, your approach of studying and acquiring new skill sets is the right path. Identify what is needed in the job market and prepare for those.
Again, not knowing the cultural nuances, it's difficult to figure out what to say. I think the best approach would be to state that you wanted to ensure that you are settled into your new married life and then look for a new job. You wanted to ensure that your professional life is not adversely affected by marriage and that the company you work for should not suffer. Now you are ready and sure, so you are looking for an opportunity.
Best of luck.