You might head south to the piece of beach more or less controlled by Club Med at Punta Nizuc (I haven't been that way in a while, but I am assuming that Club is still open,) or check if there is anything on Isla Mujeres that wold be more tolreant; topless sunbathing there might invite less hassles from both la policía and from the louts who have no clue what "sexual harassment" is. Unfortunately, sexism is quite common in Mexico yet, and "boys" who act out are less likely to be targetted by local authorites than visiting tourists foulting local mores.

By and large, Mexico is quite conservative, regardless of what you might see a tourist or two apparently getting away with in resort areas. And you wil find most Mexicans will consider topless sunbathing disrespectful and insulting to families. (Not at all my opinion, but a very common opinion in largely Roman Catholic Mexico and reflective of current mores.)

Better yet: keep it private, wait for the next trip to a more Francophile island in the Caribbean or French Polynesia. Why invite even the slightest possibility of hassles on holiday?