Wow.....I don't know you, but just from what you have told us, it definitely sounds like she cheated. The sad thing is.....these are the times you heard about - who knows what else she has done that she was able to cover up. I hope you stick to your guns and NEVER go back to her.
As far a the paternity test goes......if she refuses to get one, that means she can't come after you for child support. And that is an 18 year long commitment - whether she gets married again or not......just something to be aware of.
First, is she really pregnant? Did you ever see the test results, or the pics of the ultrasound? The only reason I even ask is because I can't tell you how many guy friends I have had who's chicks lied to them about being pregnant for some reason or another. Usually to keep them around a little longer when they knew the end of their relationship was near. Sometimes just to mess with them.
If you want to know if it's yours because you want to be involved in that child's life, then kudos to you - go to court and fight for it. The court can order a test for you......I agree that you probably don't need a lawyer at this point - unless things get ugly. And I have to heartily agree with everyone who says don't put your name on that certificate!
Good luck.....let us know how this all pans out.
And these things, while they totally SUCK at the time....usually are the things you look back on in 10 years when you are happily married to a normal chick with your nice little family and thank God that everything turned out the way it did.