Timothy Chapter 3 gives orders to those that are planning on overseeing the church body, and in those instructions an overseer or deacon are allowed to be married or have children.
So whats up with most Catholics following the man-made doctrine?
Looking at how you phrase your question, you must be fresh from a Fundamentalist / Evangelical oven, though I could be wrong. Anyway, welcome to the Forum and thanks for wanting to know more about the Catholic faith. There are many Catholics here who will be glad to oblige you.
As for your question, it helps to know that Paul says you are allowed to be unmarried in order to focus your life for the Lord (paraphrase mine). That’s the first part of the answer to your question. Unmarried deacon is allowed.
Secondly, in the Catholic Church, one needs not necessary to be unmarried to serve her, unless of course if you are eyeing to be a priest or religous sisters. But we believe that priests and religous sisters are spiritual vocations - in other word, they are God’s calling.
I share with you some wisdoms by Paul on this for your reflection:
1 Cor7
7 I wish that all men were as I am. But each man has his own gift from God; one has this gift, another has that.
29What I mean, brothers, is that the time is short. From now on those who have wives should live as if they had none; 30those who mourn, as if they did not; those who are happy, as if they were not; those who buy something, as if it were not theirs to keep; 31those who use the things of the world, as if not engrossed in them. For this world in its present form is passing away.
32I would like you to be free from concern. An unmarried man is concerned about the Lord’s affairs—how he can please the Lord. 33But a married man is concerned about the affairs of this world—how he can please his wife— 34and his interests are divided. An unmarried woman or virgin is concerned about the Lord’s affairs: Her aim is to be devoted to the Lord in both body and spirit. But a married woman is concerned about the affairs of this world—how she can please her husband. 35I am saying this for your own good, not to restrict you, but that you may live in a right way in undivided devotion to the Lord.
36If anyone thinks he is acting improperly toward the virgin he is engaged to, and if she is getting along in years and he feels he ought to marry, he should do as he wants. He is not sinning. They should get married. 37But the man who has settled the matter in his own mind, who is under no compulsion but has control over his own will, and who has made up his mind not to marry the virgin—this man also does the right thing. 38So then, he who marries the virgin does right, but he who does not marry her does even better.
39A woman is bound to her husband as long as he lives. But if her husband dies, she is free to marry anyone she wishes, but he must belong to the Lord. 40In my judgment, she is happier if she stays as she is—and I think that I too have the Spirit of God.