I believe children do essentially ruin a couple's social life, but couples should consider adding children a complete change in lifestyle and understand that before having children. This is exactly why teenagers don't tend to make great parents. Having children is 18+ year job with no vacations and no breaks.
Kids will pretty much wipe your entire social life off of the map. I just recently moved in with my girlfriend and her 11-year-old daughter, and it has been nothing short of a miserable experience. We don't do anything together, and we hardly ever get to see anyone. Everything revolves around the kid.
Yes, I do think that when you have kids you pretty much lose your social life, but it is not really a bad thing. Having kids is great, but you are going to be spending a lot of time with them, and not a lot of time running around with friends.
Kids do not ruin a couple's social life. This is because parents can then become friends with other parents, and they can create a new kind of social lives for themselves based on families rather than just going out as adults all the time. Kids' parties and sports games are one way parents can still have social lives.
I don't thionk they do unless the parents allow the child or children to ruin their social life. I think it is up to the parents to allow themselves to have some free time from the child or children what's they become old enough they should learn to be able to be able to not have to be around their parents all the time.