Some people said that she was doing it out of fear.
The fear of being alone?
She was an attractive witch once she opted for stylish things, instead of those her father gave her and she felt guilty if she didn't wear. She was brilliant, and had been top of her year in every class except Defence Against the Dark Arts, which Ginevra took credit for. If she had so desired, she could have had her pick of many wizards. It wasn't likely that she would have lived her life as a single witch.
The fear of being poor?
She had enough money from her inheritance alone to never have to worry about working.
No… Luna had almost everything she could possibly hope for. The last thing she wanted, she would get that very day. She wasn't marrying Draco out of fear. She was doing it because she loved him.