Msgr. Charles Pope address that provocative question—and provides some equally provocative answers—over at his blog:
Simply being single does not seem to qualify for what we have traditionally termed vocations. Consider first some basic differences. 1. Those who marry as well as those who enter religious life or the priesthood make promises and vows. What promises and vows do those who are single make? To whom? For what purpose? Unless they become consecrated virgins or hermits, do singles make such vows and promises? No. 2. Those who marry as well as those who enter religious life or the priesthood commit to live the life they enter stably, i.e., consistently. They do not make their promises only until something better comes along, or until something changes. Can this be said of the single? Are they not single only until something better comes along? Until they meet someone whom they will marry? Are singles really bound to live their current status stably? No. 3. Those who marry as well as those who enter religious life or the priesthood are not permitted to date others or enter into romantic and particular relationships with others. Priests and religious are celibate and the married are chaste and faithful to their spouse. Single people can date and enter into and out of romantic (though chaste) relationships at will. Are singles in a permanent and exclusive relationship? No. 4. Those who marry as well as those who enter religious life or the priesthood enter into a communal relationship, whether in a religious community, a diocese, or a third order. The single may belong to a parish, live in a certain locale, or even belong to a group like Opus Dei. But they are free to move away or cease membership at a moment’s notice if an attractive job offer comes along or some family commitment or just preference intervenes. Are singles really tied by lasting bonds to a community? No. 5. Those who marry as well as those who enter religious life or the priesthood (especially religious) live within a regula (rule), which lays out the required structure of their day, regulates relationships, and clarifies rights, duties, lines of authority, etc. What sort of “rule” do those in the single “vocation” follow? If they follow a structured life at all, is it given to them by others, or do they establish it for themselves? Are they perpetually bound or only for as long as they please? Even married people cannot date or relate to anyone they wish; they cannot simply decide to go on a tour of Europe without consulting their spouse and considering their family duties. Are the single really bound in these ways? No. 6. Those who marry as well as those who enter religious life or the priesthood are either under the authority of or answer to others. Spouses must be accountable to each other; priests and religious are answerable to their superiors and cannot simply do as they please or go where they want. Is this the case with those who are single? Do they answer or report to anyone on this earth? No. So there are a lot of practical differences that rather strongly distinguish being single from being in a promised (or vowed), permanent, regulated state under authority. It is true that some live today as consecrated virgins or hermits. But here, too, they are under the authority of the bishop and make permanent or semi-permanent promises of some sort in a way that single people do not.
There’s much more. Read it all and check out the comments, which can be lively and challenging. Lots of food for thought here.
I think a larger issue that needs to be reinforced is that we all are universally called to one vocation: holiness. We are summoned to be saints—no matter what we do or how we live or what might be our station in life. Are we doing that? And how? Are we prayerfully working to fulfill God’s plan for our lives, and doing that through our love for him and our love for others?
At the end of the day, and at the end of our lives, that is what truly matters and that is how we will be judged.