Question:
Answer:
That’s a good question, because one doesn’t want to give scandal, i.e., lead others into sin, particularly a fellow Catholic (see the Catechism 2284-87).
If these are co-workers or acquaintances with whom you are not close, perhaps the best thing you can do is simply and politely decline the invitation, although be ready to explain yourself charitably if they ask you why (see 1 Pet. 3:15).
If they are friends of yours, then I infer that they know that you are a faithful Catholic. In that light, I would explain to them they are your friends and will continue to be your friends, and because they are your friends you don’t want to affirm them in a relationship that involves some benefits of marriage but not its lifetime commitment. That is, it would be contrary to genuine love for them if you “celebrated” a relationship that is fraught with spiritual and other perils and that attending their housewarming party will signal your approval of their moving in together.
If they move in together anyway, keep in touch. Going over to their home to visit, including having a meal, doesn’t mean you endorse their lifestyle. It means that they’re still your friends—and they need friends like you. Continue periodically to encourage them to get married and to live as brother and sister as long as they live together in an unmarried state. Tell them that you recognize their friendship and that they care about one another but that living together before marriage and the associated sexual intimacy are not consistent with the current state of their friendship. You don’t have to regularly bring up these issues, and doing so can be counterproductive. They know where you stand, as your life witness speaks loudly.