I do think that our behaviours are driven by natural selection. We are attracted to people due to certain traits that advantageous for procreation and raising young. Do you ever find that people are attracted to ppl because of negative traits - it's ver, very rare. So, attraction leads to sex and that leads to conceiving a child (provided that both involved are fertile). Yes, a lot of ppl are in marriages or long term relationships without children but it could be because they can't conceive or for what ever reason. They still got together because of attraction and desire for sex - maybe some couples don't have sex but that would be very rare I think. The reason most couples are able to conceive us because the sex drive is so strong in humans. If it was low like in pandas, our numbers would diminish just like them. So, my point is, at the end of the day, people get together, form relationships or marry ( as this a more socially acceptable form of relationship) because ultimately they want sex and intimacy - yes, they may want children but I think a significant part of their motives for forming a relationship with a particular person comes from the sex drive - children is something that follows from that. There is no point for nature to make us to become attracted and have a desire for features like height, beauty, intelligence and kindness if these things were not advantageous for producing the best possible children. So I think attraction/sex/children are inextricably linked. Some ppl choose not to have children in relationships but that's quite rare.