Let's say it takes 25% of your current free time to maintain a healthy relationship with an SO (this number may vary wildly, but the broader point stands). Let's say you have about 6hrs free time on weekdays after commute and meals and such, 32hrs on weekends.

Then you have a kid or three.

Over half your total free time is now gone—~24hrs devoted to them on the weekend if your kids do 12 awake 12 asleep each day, which is typical for young kids past the newborn phase and will remain stable for several years, plus ~3hrs each weekday, for a working parent. Consider too that there will be more cleaning, more cooking, more everything to keep your house in order, but let's put that aside for now (some of that can overlap with times your kids are awake, but far from all of it).

Remember that 25% of your (original, larger amount of) time you spent keeping your relationship with your SO healthy? A little of that can come from family time while the kids are awake, but not all of it. True, you can (and to stay sane must) fudge your free time a little by having one parent do all the parenting parts of some days while the other one does non-parenting stuff, but that only gets you so far. Point is, if you don't want to end up divorced/separated/whatever, much of your now-smaller pool of free time isn't gonna be entirely free.

If it's starting to look like you've only got somewhere in the neighborhood 10-15hrs a week to do things you want to, including travel time to any activities, well... yeah, that's not far off. That's media and arts (books, movies, video games, the opera, whatever), playing an instrument, taking online courses, going to the pub, game nights with friends, sports, crafts, biking, kayaking, hiking, gardening, exercise et c. If it's starting to look like you'll only be able to keep up 2-3 of those things on the regular—yep, exactly right. You can kinda do some of those things with your kids, but it won't much resemble doing those things without kids (count on getting 10% as much done in a given stretch of time, IOW, and it won't be nearly as relaxing). You will have to abandon most of them, at least as any kind of regular activity.

I hear this gets (a lot) better when they're older, and can be left to fend for themselves a bit, but you're still looking at most of a decade lived like this, and more if your kids are spaced out.

Other things that suck: having to move to a nice suburb ($) into a house with enough bedrooms ($) in a good school district ($) OR a house closer to the city with enough space ($$, if you can even find such) and private schools because those public schools will suck ($$), OR having a parent stay home and homeschool ($$$). Medical bills can add up even for minor stuff, and insurance costs more. Having kids is basically financial suicide, and clothes/food are the least of the costs—housing, medical, and school/child care are the killers.

Travel is like 20x harder and more stressful, plus more expensive, plus you'll lose a lot of potentially fun travel time to taking care of kids, which is if anything less fun to do in Barcelona or Tokyo or a little country town in France or a campsite in the Rockies than it is at home. You probably won't want to do it much and will feel like you need another vacation afterward when you do. For that matter: staycation-type vacations will be less relaxing than they would without kids, too.

There are some cool things about it, certainly, but make no mistake that it'll be fucking incredibly expensive in pretty much every way. It is a life decision in the sense that lots of stuff you might otherwise do in your life will 100% for sure not be done if you have kids. You can't have everything, and you only have so much time and money, and kids are likely by far the biggest sink for both of those things you could (reasonably) choose to do.

Source: have three kids.

[EDIT] oh and by the way, every time you have a kid you're buying a ticket in a reverse-lottery that could end in their having chronic illnesses, brain developmental issues, et c. Imagine all of the above, especially the part about it eventually getting better, if that happens. Yeah. Ouch.