I've seen this as a recurring issue, amongst others, in some of the venting posts here (since I joined). I don't know to what degree it influences one's psychological well-being and how much it is the main issue, but it seems to play an important role. Perhaps the mechanic of it is this:
a) you are a man and your self-esteem is low;
b) women do not give you attention or you see other people in relationships, but you aren't;
c) this, to your eyes, reinforces your belief that you are not worthy (low self-esteem).
So I wouldn't say that most men are missing sexual intercourse in itself, but rather being loved by a woman in that specific, romantic way. It doesn't help that we imagine that society sees us differently when we are grown up and single, and that we imagine that if others want nothing intimate with us, we lack in value as human beings. These are logical fallacies, but still very powerful ones.
Throughout my life I've met horrible people who are in a relationship, and also wonderful people, who are single. Being single doesn't mean there's something wrong with you and it doesn't seem to be related in anyway to your general competence and value as a human being. To me it seems rather like a social practice, automatism.
I think that it is normal to believe that care and love by a romantic partner will help you regain some of your self-esteem, give you an additional reason to get up in the morning. And then there is also the need we have to love someone.