Expect to get a secondary inspection at immigration - I did when I went over to marry my American wife on VWP. I made sure I had a letter from my employer, utility bills, mortgage statement, bank statements and recent payslips. And a printout of the section of the London US Embassy website where it said it was fine to fly over to the USA to marry. Basically I got as much evidence as I could, on the basis that more is better, and organized it into a folder!
The secondary inspection was a long wait - 2 hours at ATL, but when I did get called to the immigration officer, it took a matter of minutes. That was probably one of the most nerve wracking 2 hours of my life, and you can't use your cellphone or anything in there until you've cleared Customs, so there's nothing you can do to contact your loved ones.
We applied for the marriage licence on the previous trip and that was hassle free - the lady at the courthouse just lapped up my accent.