They've been together for 7 years, which is plenty of time for all of the new relationship energy to have evaporated. Some people need mystery and variety. If you're not okay with never having that again in your life, then don't get married. I'm warning you as a person who has gone through this firsthand. You will feel endless guilt over not being able to feel fulfilled; moreover, if you discuss it with others you be shamed and criticized for your questioning and disloyalty. You will find yourself in an impossible position. You will be blamed for leaving and not sticking it out, or you will be called cruel for remaining with someone you don't love. Until you're able to precisely predict the future you cannot honestly promise to love someone for the rest of your life.

Sex, passion, desire, and excitement always fade. It's completely normal. Some people can easily make that trade to spend their lives with a partner who gives them comfort and friendship for life. But for some people, that trade is not a good deal. Society urges a particular path, but you need to do some soul-searching and be brutally honest with yourself before you make a decision.