Are there places you have been already? Are there activities you and your future bride prefer? Some would find a small eco-friendly resort in Belize to be romantic while other prefer the 5 star treatments of the Maldives. So basically, the more info we have on who you are as a couple we can be more proactive with our answers.
Traditionally romantic suggestions: Bali, Phuket, Koh Samui, Tahiti, Fiji, Bora Bora, Maldives, Seychelles, Mauritius, Boracay.
Alternatively romantic suggestions: Antartica, Game drives in South Africa/Kenya combined with travel to Zanzibar, Spending several weeks in Madagascar, Galapagos, Skiing in the Swiss Alps & train to Paris.