For sure kids are part of the deal, I had three different stepmothers after my parents divorced, and 2 of them had no kids and were okay around us, but one had 2 of her own and she hated us. One thanksgiving my brothers and sister and I were sitting in the family room and heard her say to our dad, 'I'm not feeding those effing brats'. Dad said 'those are my kids, and if you wont treat them like family, I'm not staying either.'
She let us stay but she never really came around. Once my dad spent quite a bit of money winning me a stuffed dog at a carnival, and she didnt speak to him all night.
Before I was married, I dated a man with a child, and while I knew I would never be this girls mother, I went out of my way to encourage her to get alone time with her daddy and to include her in our plans whenever possible.
My sister had a step daughter, and on Christmas and birthdays we always gave her just as much as the blood neice and nephew got. Kids are part of who your signifigant other is, and must be not only tolerated, but embraced. even if they don't like you, even if the ex is a horrible person who has done everything in their power to ensure their child will treat anyone dating their mom/dad like public enemy #1.
I remember how much it hurt, I knew my stepmom would have been happy if us kids would drop off the face of the earth.
PS~ This is not to say I don't support people who consider kids a deal breaker, I think that its great that you know what you want, and theres nothing in the world wrong with that. The idea in our grandparents generation that everyone had kids led to a lot of very unhappy people who never wanted to be parents and very unhappy children growing up with that.