When traveling under the VWP, the activities planned must be permitted under B visa.

[VISITOR VISAS
BUSINESS AND PLEASURE
Overview](https://travel.state.gov/content/dam/visas/VisaFlyer_B1B2%20March%202015.pdf)

You are allowed to visit the USA for the reasons you outlined (i.e. marriage) under the VWP program. Unlike the UK which requires a marriave visa for the purpose of marrying, the USA does not have a visa type for that purpose.