Hi Paul,
I do not think it is wrong for a Catholic to attend a protestant worship service however, it should not replace the obligatory Mass. It
cannot replace the Mass. While Protestants consider their worship service “worship” it is in no way a complete worship as what Jesus gave us in the Mass and allows us to partake in.
When I have attended another worship service of any kind I have always considered them beautiful, uplifting, prayerful bible studies. Once Protestants realize this difference in levels of worship and how the Mass plays a part in the ultimate worship then, and only then, will they begin to understand things like our devotions to Mary and the Saints and the perfect beauty in receiving the Eucharist.
The danger comes in when Catholics who themselves do not understand this true worship

and fall into the misconception of the “praise and worship” or whatever tag you place on the outward beauty of Protestant worship, as being true and perfect worship – never realizing the Mass offers so much more. Yes, there are many Catholics that do not realize these differences and therefore you should understand all the apprehension and discouragement the previous posters have expressed above.
Peace!!!