What do you think of  me having an abortion even though a family is offering to adopt?
It is strange. Why would you have an abortion when a family is offering to adopt?
I just don't want to birth a baby
Birthing a child is more healthy than aborting it. You should let the people who want it know so they don't wait for something that never comes. It's cruel to keep them in dark.
What if I have low self esteem and don't want them to raise my child?
It is a hard decision and it is up to you. But you should tell them as soon as you have decided so that they don't put all their hopes and dreams on a child that is not coming.