As a single man who would love to be a pastor, let me offer a few thoughts:
1) If I have no wife nor children a congregation can observe, how can they know I can rule a house? The only way for me to show my fitness for the job is not merely by demonstrating a knowledge of scripture, but by demonstrating a Christ-like character, and I cannot fully do that outside of a family. I have people tell me constantly that I ought to be a pastor. I've been offered a job as a pastor. I love to preach. But without a family, there is no basis to assume I'm fit for the job. 1 Timothy 3:10 - let these also first be proved. In other words, don't hire a novice. Let him prove himself in his own home first.
2) Dating a preacher? I'm not really a huge fan of modern dating, to be sure, but even more traditional courtship can cause issues for a preacher (I would assume). Imagine having little old ladies in church constantly trying to 'fix you up', putting you in a succession of awkward situations (imagine telling a dear old saint that her granddaughter is really quite pagan and unfit to be a wife, possibly even unregenerate... ).
3) A couple of years ago, before I had developed a proper perspective of dating, I had met and really, really been smitten by a young lady. When she told me she wasn't interested in seeing me, I took it rather hard - and the next mo0rning, the music director of our church called me to ask if I'd fill the pulpit for mid-week service. I had to decline. My focus was gone. I'm sure married pastors deal with this regularly, but to be honest, I didn't enjoy the prospect of preparing a message while downtrodden over things not working out with this young lady.
4) How can I fully articulate Christ's love for the church if I don't understand what it means as a husband to love a wife? If marriage is meant for us as a picture or type of our relationship with Christ, I'm hindered in ability to convey the beauty of this relationship, not having any practical experience in it.
5) Consider Adam: He witnessed creation, he named the animals, he was on hand for some of the greatest works of history, but he never once spoke anything worth writing down until he was given a wife - at which point he spoke boldly! Theretofore Adam had been speechless, but in Genesis 2:23-24, Adam now boldly proclaims righteousness to all of creation. For the first time ever, Adam had a real live flesh-and-blood helpmeet to support him, even to validate him. Eve completed Adam, in a sense.
6) An extension of point 5 - a single, never-married preacher will rarely if ever be taken seriously to the same extent as a married man. Having a family, in one sense, validates a ministry, validates a message. People can look at a married man and say 'well, at least his wife beleives in him - let's give him a shot.'. An unmarried man doesn't get that.
Al Mohler described marriage as the 'crucible of saint-making' in a sermon he did a few years ago lamenting the unwillingness of many twentysomethings to 'grow up'. I think he's right. As a single person, there is NO way I'll ever be sanctified in certain areas. I'm self-employed, money isn't an issue, I don't have a wife and kids to worry about, I'm not forced to put up 24/7 with someone who I love yet drives me batty with their quirks......a pastor NEEDS those things to help conform him, through trials, to the image of Christ. Titus 1:7-8 speaks of these things that are (I hope) products of a Godly marriage.
Anyway, that's my $0.02 on the issue.
<---single, waiting as-patiently-as-I-know-how for God to deliver a wife.