If you have served the petition already I think you have to get the Court's permission to amend it, which would cost you a small fee but is doable. In not then change it as the reason for divorce makes no difference in any practical sense. If he felt strongly enough about it and you put adultery he could try to defend the petition (he wouldn't be advised to do that) which would cost lots and take a lot of time.
Dp is a divorce lawyer and just mentioned that as he walked through the kitchen. I'll ask him for any further information if you want but he's gone out.
Misscathcart. When I was divorcing my ExH, he refused to acknowledge receipt of initial papers (although I had to go through a lawyer). In the end, I had to pay for someone to physically put the papers in his hand (can't remember what you call those people!!) but my lawyer needed proof that he had received the papers in order to proceed with the divorce, so that was the only way to do it.
Rubles is correct in saying that if the adultery petition has already been issued, you will need to amend it and pay a fee. If it has not been issued, then the most sensible course of action would be to issue a petition based upon his unreasonable behaviour (obviously his affair could be one of the reasons). You normally need at least 4 or 5 different allegations although it does vary from court to court and the final one must be fairly recent, ie in the last few months. With an adultery petition, he would need to actually admit the adultery so if he won't agree, you will be stuck. Whereas with a behaviour petition,he doesn't need to admit it. If he fails to complete and return the acknowledgement of service form to the Court then your next step would be to have him served personally (process server/private investigator) and they will then file an affidavit (statement) of service so the divorce could proceed. It is unlikely that he would take the step to actually defend the divorce as any lawyer would want at least a grand up front to represent him and at the end of the day, defending a divorce is really only a delaying tactic and ultimately it will go through. good luck