I'll toss out a few random thoughts. Today is my oldest daughter's 9th birthday, which marks the half way point on her journey to adulthood. Bittersweet. The bitter is that half her time living with us (as a child) is gone. Sweet because she's transformed from a pee/poop/eat machine that cried all the time into a rather smart, crafty, cunningly complex little creature that can think and rationalize independently, has her own interests and ideas, and amazes us at times (both good and bad).
We also have a seven year old and a two year old.
I managed to retire recently after reaching FI at 33. Although "stay at home dad" might be more appropriate for the next three to sixteen years since I spend at least a few hours each day engaged in some parenting role and it does prevent me from pursuing other interests (reading, writing, relaxing, drinking beer with friends, etc).
In my experience, kids aren't that expensive, although we have been creative and made compromises to keep costs down. We do plan on spending low to mid five figures on each kid for college, and possibly a full ride if our portfolio grows as projected over the next 9 to 16 years.
Some might say we should spend way more on our kids. I figure we do "enough" to enrich their lives as is (travel, camps, entertainment, activities, love from parents, education, safe environment, etc). But there will be some parents who would criticize us for not enrolling our kids in more activities, or buying a boat or vacation house or large SUV. All things that some people associate with "the best way to raise kids".
From virtually any kid's perspective (if they were to be overly contemplative), they would prefer to exist and be a child of a frugal parent than not exist at all. Don't fall into the trap of binary thinking. There are options beyond (A) don't have kids because they are expensive and (B) have kids and spend tons of money on them because you have to or else they'll end up fcuked up later in life. Lots of the best things in life are free or very inexpensive.
I enjoy my kids a lot but it's a lot of work, too. No way to sugarcoat it! The good news is kids are a great way to keep you busy in early retirement. The bad news is they take up a lot of your time in early retirement.
As for wanting kids and its impact on your relationship with a significant other, I'd say you need to reconcile differences early on. If someone adamantly wants kids, and you definitely don't, then there's an issue. I'd say a bigger issue than differences in religion (you can totally phone that one in if you are forced to convert, for example!). If you are ambivalent about kids, and your SO really really wants them, hey go for it. If you are against kids, it would be tough to sign up for an 18+ year sentence of doing something you don't want to do. That's roughly a quarter of the average human lifespan.
I never gave kids much thought until I married Mrs. RootofGood. She wanted six. We had 3, and she's happy now (they are a lot of work!). I'm happy having kids too (probably 90% of the time). Although I bet I would be just about as happy without kids. I don't mean to say want to get rid of my kids, as that's absolutely not true. Rather, if I never experienced the joys and trials of parenting, I wouldn't know what I was missing. In my alternative kid-less life, I'd have a lot more time to party it up and fill my limited time with other valuable and rewarding pursuits.
But standing here today, halfway through my oldest child's period of youth, I would miss each of them (even the snot-nosed 2 year old) very dearly if they suddenly disappeared from my life.