Ask Lemmygrad

There are two different viewpoints on this. One is the one you are mentioning which holds that having children is something selfish that you do for yourself. There are a number of arguments for this view such as the personal fulfillment many people get from raising a child, having more people around you who love you and who can take care of you when you are older, and of course getting to "pass on" something of yourself. In that sense it makes sense for some people to view it as selfish.

The opposite view however would argue that it is actually selfish not to have children, since children are necessary for the perpetuation of society. This too makes sense: By having children you are doing something good for society, adding to your community another person who can help make that community better while potentially (depending on how much of a burden you view taking care of a child as) giving up some of your other goals in life as you are sacrificing your own time and resources.

Both views exist and i don't think we should be arguing about who is right and who is wrong. Ultimately this is a personal choice and it has no relation to being or not being a good socialist. The duty of a socialist society is to help with all possible means those who do choose to have children, providing them with all that they need to raise a family, while also respecting the choice of those who don't and making sure they too are taken care of by the larger "family" of society.

Being against people having children and viewing it as a selfish act in of itself, devoid of any context inherently upholds the view that humans shouldn’t exist. The whole foundation of the ideology has to led to that position.