How demanding a job it is depends on your involvement as a father. If you choose to be involved, it's tough. If you choose to be the "shut up son, I'm reading the paper" dad, it's not. But then you'll discover why most divorces are initiated by women.
Just my opinion, but I do you really want kids? You sound like you are more concerned how it will affect your life. My opinion doesn't mean jack since I have none, but I always assumed that the life you're used to comes a close second to the well being of the child?
Yes and no. Kids do come first because... well, they demand it. But you put yourself, first, too - because otherwise you can't help the your kids. It's like how in an aircraft if you're with a kid and the oxygen masks drop, you put yours on first - because you can't help your kid if you pass out. Same in all first aid situations.
So you have to take care of your needs to a degree, simply to be able to be a functioning and useful parent. A classic example is the screaming baby. You've changed them, fed them, sung and rocked them for half an hour - and they're still screaming. It's like fingernails on a blackboard. You're going nuts, you have a brief thought of shaking the child in frustration, or even smothering the child to shut it up.
That's the time to put the child down in their cot and go outside in the fresh air for ten minutes. As my paramedic friends say of patients, "If they're still screaming, they're still alive, they can wait." Your sanity or losing it in those ten minutes is a danger to the child, whereas being alone for ten minutes in their cot is not. So you put them down and go outside for a bit to swear or cry and breathe deeply and calm down.
From this it extends further. Sometimes you just want to be able to have a crap alone without a small someone wandering in to say hello. You need to get out of the house and have a conversation with someone which is not about babies or the consistency of their poo. And so on.
It's not really that the work is
hard, more that it's
relentless. There's no scheduled coffee or meal breaks, you're available 24 hours a day.
What you quickly discover is that there's a reason that God or evolution or whatever made two parents. It's too much for one. There are times when you need to step in for your spouse, "I'll take care of this, you go and have a cup of coffee or a lie down, I'll come wake you at dinner time." And they'll need to step in for you.
A lot of this can be planned. You change your work so that you're doing four long days instead of five shorter ones, this gives you a Friday alone with the kids so your spouse can go out all day alone. Or you commit to coming home from work by 4pm on Wednesdays so the spouse can go out to volleyball.
Having children will absolutely impact you and your life. You are not going to have ten hobbies and wake naturally every morning after a solid eight hours' sleep. If you organise yourselves well, you will have one hobby and wake naturally every second morning after a disturbed sleep where you just rolled over and went back to sleep - the other mornings are where you're the one who got up in the night and early. That is, you take turns. But again, this has to be planned.
As a personal example, each weekday my wife gets up with the kids 6-630, and I stay in bed until a bit after she leaves the house. I then get up and sort the kids out for the morning with breakfast and the older one's schoolbag, we take him to school at 830. Then it's back for the day with the younger one, a toddler. I do housework tasks spread through the day - but not ad hoc, planned. Monday, Wednesday and Friday I vacuum and clean all benches and surfaces, Tuesdays I do fruit and vege shopping and Thursdays supermarket shopping, and I have stock lists to tick off, stock lists which relate to the menus, since every Monday we have chilli, Tuesday we have soup, and so on.
Tuesdays and Thursdays people come to my garage gym from 4pm, so my wife finishes work early and picks the older kid up from school and is home by then. She'll then heat up the dinner I made earlier in the day so we can eat at 6. This means she's later home on Mondays and Fridays, making up the hours she was short Tue/Thu. Wednesday night I go out to the city to play games with friends. Friday night is shabbat dinner, we often have people visit us then, if not then it's a night home together for my wife and me, we'll play a game, watch a movie, or listen to an old radio play.
And so on and so forth. Being a parent is nothing mystical and magical. It's a job. You do your job better if you organise it well. This doesn't mean being the "tiger mum" and driving your kid around to six different activities a day. It does mean planning and scheduling to a degree, and spreading the workload around through the week and between the couple reasonably.
Because if you don't plan, then one person - usually the woman - ends up doing everything, and doing it ad hoc. And things are chaotic and there's screaming and crying and not just from the kids. If you make your spouse feel like a single parent, there's every danger they'll decide to make it official.
It's a job, and properly-done it's a job like any other - it'll be mostly tedious, sometimes frustrating, and have moments of joy. Because it's a job involving your own children, the frustrations and the joys will be greater than those of any paid job.
People make a big deal of being a parent and toss a lot of emotional baggage at you about it, especially at women. As a man you get to miss out on all that bullshit about breastfeeding vs not, childcare vs not, whether to ever give your kid lollies, how to encourage them to walk - believe me, this trivial shit is the subject of arguments more bitter and vicious in mother's groups than the Israeli-Palestinian dispute has ever had - but you will get it second hand.
Having children is neither selfish nor unselfish, any more than working as a coder vs working as a teacher is selfish or unselfish. Everyone's work contributes to society generally, and gives them benefits, too. Everyone likes to think their job is the most important job in the world, but at the same time they have days where they resent someone or something about the job and its demands on them. Toss aside all that self-indulgent nonsense: it's just a job. But you should try to do your job well. Have some professional pride.
In the end, it's just a job, a job which you can do well or do badly, a job which is mostly tedious, it just happens to be the job with the greatest joys and greatest frustrations of your life.