If we are going to play with someone new we would prefer to know their rules upfront and we ask early on in our conversations. We like to know the other couple has discussed their boundaries. There's no reason to keep your friends in the dark about what your rules are. It makes for awkward situations. Keeping your reasons to yourself is fine. Communication in swinging is really important between you and your spouse, but also between you guys and the other couple or single.
Good luck!