You do get special underwear by being married in the temple, but also can get it if you aren't. That part is called receiving your endowments and is most often done the day before the wedding or before a Missionary goes on his Mission. You have to be over a certain age and deemed worthy. You have to have what is called a Temple Recommend to get in. It is renewed every few years and requires an interview with the Bishop and Stake President.
The first thing you have to do to be worthy is pay 10% of your income. It used to be a personal decision as to whether that was on your Gross or Net Income. At the end of every year they have a tithing settlement where you go in pretty much settle up with the church and get your tax information. It was only about 10 or fifteen years ago when they started saying you had to be a full tithe payer to get a recommend. They must have been low on cash.
So if you attend church regularly, don't cheat on your wife or husband, don't drink or smoke, and you are over the age of 18, you can get a recommend. Which means of course that a girl getting married in the temple, can't have her younger under 18 family members attend or anyone who doesn't have a recommend. I've sat in the lobby a few times with a room full of siblings. Anyone can go inside a new temple before it is dedicated.
The temples anymore are McTemples if you ask me. It used to be that they were pretty grand, but now they are just really big churches. I haven't been a Mormon for over ten years, so some of this might be dated. No I wasn't desfellowshiped. I threatened to sue if they didn't take me off their records. Thank heaven's there was the Internet with a bunch of people who had already paved the way.