In light of the ecumenism following Vatican II, are Catholics now under serious obligation (that is, binding under pain of sin) to skip Sunday Mass once in a while and worship instead at non-Catholic churches?
“Required”? “Not allowed,” you mean! A Catholic may not “worship instead at non-Catholic churches.” To do so is to violate the First Commandment. (This is to be distinguished from engaging in, say, an ecumenical prayer service.) It is mandatory, under pain of mortal sin but allowing for exceptions arising from necessity, for every Catholic to attend Mass every Sunday (or Saturday evening) and on holy days. Catholics are under no obligation to attend non-Catholic services. Such visits are purely voluntary and may be undertaken so long as no scandal is given. Vatican II’s Directory on Ecumenism allows some participation at such services in common responses or hymns that are not in variance with the Catholic faith.