I have been married for almost two years, but only lived with her for one as a married couple. I am going to a lawyer for a divorce and just recently received a summons for childsupport. She has been living with a man whom she cheated on me for eight months now. During these eight months I have not been bothered for childsupport till now. My question is , is it my responsibility to pay childsupport although the child is not mine and she has been living with this man for the eight months.

If the child was born while she was married to you, in almost all US states, the child is legally yours. Did you after the birth of the child contest the paternity. So you have to deny the child being yours and demand a DNA test. In most ( but not all) US states once you prove in court the child is not yours, by DNA test, you should not have to pay child support.

Some US states have time limits to challenge DNA and I believe a couple you are the father, since you were married when child was born

If the child was born before you married and you did not adopt, then you will probably not be held accountable for support. Depends, though, on local laws. Doesn't matter who she has been living with just what your relationship to the child is.e