Their website continues with the requirement to "participate in Opus Dei's mission." Again, my answer is that I have my own mission to carry out! I don't have time to do things for Opus Dei in any capacity. They can go take advantage of someone else to do things for them because I'm definitely not Opus Dei material.
Opus Dei says that "joining Opus Dei requires a supernatural vocation. It's a personal call from God to place one's whole life at His service." Who has time to do that for Opus Dei? (We're all called to put our whole lives at His service, not Opus Dei's.)
I had a personal supernatural vocation to be a mother to my children. It was the one time in my life that I knew what I was for. It was my supernatural calling to change their diapers, feed and bathe them, guard them from danger, take them to school, teach them about God. Who was going to do all that while I went around and worked for Opus Dei all the time?

