I think its a combination. While I was growing up I didn't want kids and didn't want to get married. My background had more to do with that, so more nurture than nature.
Here I am happily married for almost a decade, and as a couple going on 15 years. Took my spouse about 5 years for me to warm up to the idea of marriage, and about another 7 to have a kid. I just wanted to get all I wanted to do as a couple, and make sure we were compatible for the long haul. We didn't have a kid because it was the next step, we did it because we wanted to share what we have with a miniversion of ourselves, not to mention we wanted to see what our genome was going to conjure up. Extremely corny, I know. If it weren't because I'm a huge control freak, my bio clock was ringing big time I probably would have had her earlier. I had the pets that are treated as family, but it wasn't as fulfilling as it was in my early 20s.
But by the time we planned to have our daughter we had all our ducks in a row, the house, savings, childcare, insurance of all types, stable jobs. And I am grateful we did, if it weren't for insurance, and savings we couldn't have lived in the hospital as long as we did on FMLA without accruing debt during my daughter's diagnosis/treatment.