Is it reasonable to worry about lack of potential for dating in postdoc positions?
How is Academia different that any other job? You work for a number of hours a day, and then you are free to use your time solving jigsaws at home or socialising with other people.

Perhaps the academic environment has the advantage of multiculturalism. In most industries, the large majority of the people are from the country; but at universities you can find much more diversity, and thus, easier to find people you feel more comfortable with.

Lastly, in large cities there are, statistically, more potential "suitable partners", but they will be more difficult to find. As a mathematician, you may want to model it as an Erdős–Rényi graph, for the fun of it.

In the end, I think either path will have comparable a priori chances of finding love, so this should not be your main criteria to decide what to do with your life. Being happy with what you do will make you a nicer person, and have a larger impact that the city you are in.

And, to put at ease your concerns, most professors I know are married, and they certainly went down the postdoc path.