Plus restricting the number of people involved in a legal contract isn't protected anywhere in the constitution. You could argue freedom of association - but their ability to associate isn't being restricted. It's just their ability to enter into the legal agreement we call marriage.
If a theater is sold out, is it a violation of your rights to be told they can't let you in because it would violate fire codes? Nope.
Again, consenting adults can do what they like. But the government isn't discriminating by limiting legal marriage to two adults.