Wow that's got to be a world record getting nagged about having a second when your first is still in the womb!!!! The subject of children often brings out the worst side as well as the best side of people's natures and you will find throughout your child's life that you will encounter people behaving in a way that's intrusive, judgemental and often downright rude. The upside is you've established early who your main culprits are likely to be!! Some people will always feel they have a right to tell you what to do with your life and very often these people are holding on to outdated beliefs or trying to validate their own life choices by enforcing them on other people. Unfortunately they often use very extreme, emotive language as you have discovered. Common sense should tell you that the word "cruel" is entirely inappropriate and their attitude says more about them than you. You are doing something a little different to what they perceive as the norm and they are uncomfortable with it, which is their problem not yours.
The truth is one-child families are very common and make up almost half the family units. The false perception of only children as being lonely children is outdated as it has been disproved time and time again.
You will discover when you have your child that there really is no magic formula to guarantee a happy, healthy child. There are many different ways of parenting and many different types of child. Having siblings does not necessarily enhance your life and being an only child doesn't necessarily mean they will be lonely. The best thing you can do is give your child a happy, secure and stimulating environment to grow up in so they can be confident in who they are and deal with whatever life throws at them.
Dh and I have decided to stay with one and it feels so much better to make that decision. I've found motherhood very hard work and financially and mentally I just can't face it all again. My daughter is at a nice age now (2) where she can eat, sleep, walk and talk independently and I'm enjoying her a lot more now the baby stage is all behind us. We can give her more and do more with her as she's an only.
What I'm saying is you need to have the confidence to make your own decisions about your family and stand by them. You will need to build up a resilience to dealing with the opinions of others because they can go on as much as they like. At the end of the day it's your life and you know best what's right for you.
I wish you and your little girl all the best and I'm sure she will be very happy and very loved - nothing cruel about that!!