Of course, it is their choice. The question is: Is it a good choice. A sin is not merely an affront to kid, it is, humanly speaking, a misstep. Misstep on the stairs and you may break your neck. If enough people break their necks, then the hospitals will be full. If enough people make wrong choices about parenting, then society suffers. If enough choose not to have children, then society may cease to exist.
I honestly don't think that will happen. If that happened then the Bible would be wrong then. Think about it. That's not how it all ends. I'm 100% pro-life. So I think all persons capable of producing children should weigh that decision carefully BEFORE having them. Unfortunately some give more consideration to whether to get a puppy than having a kid.
If a persons temperament is not suited for raising children then it is wise not to have them. But that person may be a fine spouse to another person of the same temperament. Let them marry and live in peace. I pointed out examples before on this thread and one of them was medical reasons which is why we did not have a kid together.
Now I could have had a kid had I simply not married my wife and married someone else. I knew before we married we could not ever have kids. But I did not go seek another woman for my wife because this woman is the one GOD through many intervening events in both of our lives placed me with. Take it anyway you wish but I was told to take her as my wife. I had more Why's for GOD than I can now recall but the answer was It's going to be OK. No kids by my first wife of 3 years and none from my present one of over 20 years.
It wasn't GOD's plan. GOD saw the future I could not see. He saw my wifes daughters and their need for a stable home. He saw the kids they were to later have. Later on I understood the Why's.
I wish so very much the things that happened had not come to pass. By that I mean I wish she was healthy as well as myself and we would have possibly had two or more kids ourself. But GOD had his reasons why we didn't and some I'll likely not know in this life. The doctors said no kids and I responded accordingly to prevent it which in some churches is also a sin. Better for her to die perhaps then or the child have severe health issues from her medications? Some preachers need to let GOD be GOD and stop trying to replace him with church doctrine.
Now I'll put the same condition on myself I placed on married couples whom GOD has placed on their hearts to have kids but don't. Had I walked away from my then girlfriend and now wife it would have been sin. Why? Because it was placed on my heart to marry her and by not doing so it would have been sin.