Congratulations! In case it's not clear, here is what you should do to sit together and otherwise increase your chances of having good flights:

1. Call United and get the Record Locators/Confirmation Numbers/Passenger Name Records (three different terms for the same thing) for both United and the Star Alliance partner airline(s) you'll be on. The United PNR is usually different from the partner airlines' and you'll want the latter for dealing with the partners. United should also be able to give you the partners' phone numbers, though of course you can find that online. (Also make sure you have ticket numbers, one for you and one for your wife/husband, as these are different from PNRs and are especially important to have to make sure the tickets are booked properly and if there are problems down the line. They should be on the emails you've received from United and viewable online at the United site, but just make sure about this. PNRs can be assigned even if there are no ticket numbers due to United occasionally improperly booking the flights.)
2. PNRs in hand, call the partners and explain that you'd like to sit together. (Of course, do this with United as well if you're on any of its flights.) I believe a few Star Alliance airlines don't assign seats far in advance if you don't have United status, but as far as I know the Asian partners are good about this.
3. As indicated by my parenthetical comment in #1, United occasionally screws up in booking partner airlines' award flights due to problems with its reservation system or customer service reps, so when you call the partners make sure to ask if everything looks fine and whether the flights are properly ticketed.
4. Screw-ups can also occasionally happen even after flights are ticketed, due to things like airlines' schedule changes. The impact can range from misconnections to losing your seat assignments. So check your reservations online or by calling the airlines in a couple of weeks and again a few days before you go, to make sure everything is still in order.
5. Don't worry...despite what I've said the ticketing problems are the exceptions to the rule. But if you do run into problems, come back here to FT for advice!
6. Email the hotels at which you're staying to very nicely and politely let them know this is your honeymoon and any little complimentary recognition they could provide would be greatly appreciated. Don't demand or expect anything; but there's a small chance that you might get some extra little perk in recognition that this is a very special trip.
7. Have a great time! Thailand is great! As you may know, depending on what part of the country you're in you might be catching the rainy season. (E.g., it might be rainy in Phuket but not in Kho Samui in August.) But it's a wonderful, friendly, beautiful country with great food, regardless.