I've been asked to comment on the question of whether the Catholic Church teaches (and if so whether it is correct to do so) "that only the celibate can devote himself completely to God while this is not possible for marrieds because they have other responsibilities."
Consecrated virginity has always had a place in Christian spirituality. The most obvious discussion of it in the New Testament is probably
1 Corinthians chapter 7, where Paul discussions a number of concerns surrounding marriage and said famously:
I should like you to be free of anxieties. An unmarried man is anxious about the things of the Lord, how he may please the Lord. But a married man is anxious about the things of the world, how he may please his wife, and he is divided. An unmarried woman or a virgin is anxious about the things of the Lord, so that she may be holy in both body and spirit. A married woman, on the other hand, is anxious about the things of the world, how she may please her husband.(1 Cor 7:32-34)
Paul's discussion makes it clear that some, including himself, remained single during Apostolic times in order to concentrate more fully on the Lord's work. In the coming centuries, the great traditions of Eastern and Western monasticism would spring up, many examples of which survive down to this very day.
Paul assures his readers several times that it is not sinful to marry, and advises any who don't feel up to celibacy to get married so that they won't find themselves tempted to more freewheeling solutions to their desires. But if people are serious about holiness, their question is not "is it sinful to marry" (and really, who could imagine that it was, given that the Church constantly uses the image of husband and wife for the love between Christ and the Church) but "is it less holy to be married than to be celibate" or perhaps more indignantly, "are you saying that I'm not holy just because I'm married?"
As with any good question, the correct answer is not necessarily pat. Obviously, being celibate does not itself make someone holy, though I don't deny that at certain times and places some people may have imagined such.
Still, family life, for all its blessings and channels to holiness, can make certain approaches to spirituality difficult. Case in point: a few months back I helped get a group off the ground in our parish that says Vespers four nights a week (M-Th) at a timeslot that's basically right after work. It's a very peaceful cap to what's often a rather crazy 10 hours of my day, so I've really been enjoying going down there, and feel like it's added a much needed spiritual pause in my schedule.
However, much though I love the Divine Office (and really admire the way it structure the whole day of monastic communities) things keep happening to underline the fact that is an element of spirituality which is not always a 100% fit with family life. For instance, at first, MrsDarwin and I were trying to go together. However, this meant taking the girls (ages 5, 4 and 1.5) and this proved such an abject failure that our associate pastor (fairly tactfully) requested that we avoid it in future. It's easy to take a kid out or hush her in the middle of mass. However, when half a dozen people are reciting psalms antiphonally in an otherwise silent chapel, you can't step away, and you can't hush the kids. Much though it annoys me when people act like children don't belong in church, I had to admit to myself after those first couple tries that you just can't take kids this young to Vespers. They don't understand it, and hushing them isn't practical.
As for the sort of schedule of all eight hours of the Office, daily mass, spiritual reading, etc. that monastics do: not only would it not fit well with family life, it would be an active abandonment of your vocation as a parent to try to live like a monk or nun. As parents, we participate in God's creative power by bringing new souls into the world, but accepting that vocation means accepting an active, not a contemplative, life.
Traditionally, the contemplative life has been seen as the highest form of Christian spirituality, on the theory that it is the most like heaven: the life to come. In the last fifty years, many people have come to frown on that view, seeing both active lives (whether parenting and working or ministries devoted to active service of others) and contemplative lives as "separate but equal" means of holiness.
Personally: I'm a fairly traditional kind of guy in a lot of ways. It does seem to me that the contemplative life is more similar to the life to come, and thus a powerful road toward holiness. However, I think that understanding needs to be balanced with an understanding that we are not currently in the world to come. We currently live in an earthly realm, and as such most of us need to spend most of our time focused on basic things like food, shelter and reproduction. We're meant to do that. That's why we have bodies.
So while I think that the contemplative life lived out by celibate monastics is more a window into heaven than my own, I'm not worried about it. All of us, in our different vocations, are living out parts of the Christian journey, and I don't think it's important to worry about "higher" and "better" paths so much as to live out the path you're on as well as possible.
Once upon a time, back in college, my roommate when to a Catholic "vocations fair" where numerous orders had come to get recruits. He saw a poster that said "Are you called to marriage, the priesthood, or the consecrated life" but misread the last as "concentrated life". Seeing this, he thought, "Well, you really can concentrate on things more if you're single. Maybe 'concentrated life' is a good phrase for being single." (The mis-reading, when discovered, was less interesting. But he eventually found a "pasty white blond" to marry and didn't have to worry about the issue anymore.)
A while back when MrsDarwin and the girls went off to visit relatives for two days, I got a taste of the "concentrated life". Wow. There is a lot of time if there's no one else in your house. It could be very peaceful. You could become very, very dedicated to and good at some hobby or duty in all that time. (Personally, I wasted it all on watching anime on the computer and drinking beer.)
I think this is why the use of consecrated virginity shows a lot of wisdom. You do have a lot more time to devote to God if you aren't dealing with a career and a family. However, you also have a lot of time to fall prey to laziness or gluttony or envy or whatever other collection of vices you're prone to. Celibacy gives you a rope, but it doesn't guarantee that you'll pull a wagon with it rather than just hanging yourself. And as with all things, the more you have, the more is expected of you.