"It is inconceivable, unfathomable, that it would be Our Lord's will that a young adult, who is dedicated to getting closer to Him and is perfectly able to accept the marriage/family vocation, rejects this vocation and chooses to remain 'single.' "
I did not invent this statement. It is a statement someone actually wrote. I do not agree with this position. Please note: I do not agree with this position. However, I think it is always important to do one's best to understand everyone, including those people with whom we disagree. Hence, I would like to make some remarks on why it might seem selfish for someone not to marry, and yet without becoming a priest or religious. Before getting upset and posting a comment along the lines of: "How can you dare say that single persons are selfish?", please take the time and to care to actually read what I say in this post, and to pay attention to what the actual purpose of this post is.
One person may marry because they think they will be happier in marriage. Another may refrain from marriage because they think they will be happier without marriage. What's the difference between the two cases?
Though in a sense the motivation is the same, there is a difference between these two. What if we described the two situations in this way: some resolve to love because they think it will make them happier; others resolve not to love because they think in this way they will be happier, that love would not make them happy.
I'm not saying that this is the reality of the motivation of all those who remain single by choice. But I would say that as long as it is purely negative, that is, as long as they are single simply because they don't want to marry, and don't intend to use the freedom offered by the single life to love God and/or their neighbor, they are not embracing the single life as a vocation, and are being selfish.
And yes, those who marry without intending to love their spouse through their marriage are also being selfish, and even more so than those who don't marry at all.
When someone doesn't marry, and becomes a priest or religious, then people see that there is some other mode of love that they are devoting themselves to; if they don't see this in another person who remains single, they may see him as selfish. Of course we cannot judge the heart, but if is true than the person remaining single isn't giving themselves in love to any one, he or she is in fact selfish, and that appearance is correct.
Much more could be said about the question of happiness. Everyone seeks to be happy, and no one seeks to be unhappy. St. Augustine and many others have pointed this out many times. The difference is precisely in how we seek to attain that happiness: do we seek to attain it in God, through giving ourselves in love, or do we seek to attain it in a good lifestyle, through piling up things, time, etc., for ourselves? The first way is good, the second way is not.
See also the post "Single vocation?" on whether there is a single vocation.