Well, despite what you may think, you can’t just automatically excommunicate them.
Canon lawyer Ed Peters notes one case in particular:
Illinois Governor Pat Quinn, a Catholic, is pushing the ‘gay marriage’ agenda in the Land of Lincoln to the obvious and rightful chagrin of Catholics faithful to the natural law (not to mention, to Church teaching) on marriage. But in calling for Quinn’s excommunication, specifically his automatic excommunication, the Rockford Pro-Life Initiative betray their lack of understanding of Church law and succeed mainly in diverting attention away from Quinn’s grave failure to perform his civic duties properly and toward the intricacies of canon law. Support for so-called ‘gay-marriage’ is not an excommunicable offense (see Book Six of the 1983 Code) and it cannot be made one except in accord with Canons 1314-1318, none of which has been put on the table. Meanwhile Canon 1369 gets ignored again, and for that matter, Canon 915 is overlooked as possible response. May I suggest, once again, that people who want to invoke canon law for this cause or that, should engage in some elementary canonical research before pronouncing away.