I am filed for divorce. My wife's second child - born while we were married - is not biologically mine. I signed a denial of paternity and did NOT sign the birth certificate (we live and the child was born in Texas). The divorce proceedings are getting heated now and it really chaps me that she is going to "let" the court force me to pay support. AND she still has contact with the biological father.
What are my options? I was told as the "presumed father" I would be MADE to pay support -- even though I never signed the certificate AND signed a denial of paternity. Do I have the right to go after the biological father for back child support?

In Texas, as in all US states, when a child is born to a married couple the husband is presumed to be the father. Not signing the birth certificate, and signing a denial of paternity doesn't do anything unless you go to court to have someone else declared the legal father.

Depending on the age of the child and how you have treated the child, you could in fact be declared the legal father.

So I would suggest you hire a lawyer to advise you how to get out of this predicament.

I would also like to point out that if you have treated this child as your own until the marriage fell apart, then you are a cruel person to deny the child now. It is not the child's fault your marriage is ending, and to punish the child by denying your love and support is a pretty low thing to do.

I would guess you obviously do not have an attorney, you need one,

Can you be forced to pay, yes, you are still legally the father and unless you properly file in court to contest the paternity and ask for a DNA test, you will be paying.

So before you say you can't afford an attorney, can you afford 300, 400, 500 a month or more for support for the next ? How many years??

You can not afford not to hire an attorney.