"Would you be down" is a colloquial phrase for "Are you interested in..." or "Would you like to do...". This is mainly used in very casual settings between two friends that know each other well. "save ourselves for marriage" means, I can only assume, to abstain from sex and to wait until marriage. However, I could be wrong here. Depending on the context, it might mean to simply remain faithful to each other in a relationship. Bonus: "Like" is a filler word that isn't grammatically correct here, in other words, you can omit it and the sentence still survives. (In Elementary School I would write "like" in the middle of my sentences, and my teachers would always correct my mistakes lol)