My name is XXXXX XXXXX I'd be happy to answer your questions today. I'm sorry to hear that this happened, and I can certainly understand why you don't feel that you should have to pay alimony.
First, child support
and alimony are two different issues. Assuming that the children live with her, under Virginia law, child support terminates when the child turns 18, unless he's still in high school. In that case, you would pay until he turned 19 or graduated, whichever came first. If you agree to pay college costs, then you could be required to pay support for longer (but there is nothing in the law that requires you to agree).
If you are ordered to pay alimony and you do not, she can have you held in contempt of court. That usually starts with fines, but a judge can choose to put you in jail if you have the money and simply refuse to pay it. Under the full faith and credit clause to the U.S. Constitution, any state in the Union must enforce a court order from another state. That means that, if a Virginia judge issues a warrant for your arrest, and they find you in another state, you will be held in jail until you can be transferred back to Virginia and held accountable there. That's a situation you probably want to avoid, if at all possible.
With that said, if you can get a divorce on the grounds of adultery, Virginia Code, Section 20-91(A)(9)(c) suggests that you do not have to pay spousal support
. To prove adultery, it is usually enough to show intent and opportunity. Also, this isn't criminal court - you don't need to prove adultery beyond a reasonable doubt. You need only show it by "clear and convincing evidence." Hopefully, whatever the private investigator gave you that convinced you will convince the judge as well.
As far as manifest injustice goes, that tends to apply where one spouse has special medical needs, is disabled, or cannot work for other reasons - say the two of you were much older, and she had been out of the workforce for 40 years. In this case, you can argue that, because she is physically capable of working, she can get a job to meet her needs, especially in light of the fact that you are giving her money to support the children. On top of that, when the last child moves out of the home, you can go back and request a modification on the grounds that she is no longer raising children and, therefore, there is nothing to prevent her from getting a job.