As a parent, I can say that time, money and to a greater or lesser degree, freedom are the things that you give up when you have kids.
Time, in the sense that children, especially young ones, are dependent and just daily maintenance activities and chores take up a lot more time than if it’s just you and your SO.
Money- this one’s self-evident, but it’s not just incremental costs like food, electricity, etc… but whole new categories- day care, mother’s day out, kid shampoo, diapers, etc…
Freedom- even if your kids aren’t actively doing anything requiring your time, you’re still not free to just bail and go have dinner with your SO, unless the kids are old enough to take care of themselves. You have to arrange a babysitter, and there’s another cost you have to weigh against the utility of having that kid-free night.
All that said, children are really more than the sum of the parts- going into your 2 1/2 year old son’s room early on a Saturday morning and hearing a really excited “HI DADDY!” and instantly getting a hug makes it all worthwhile.