ITA!
My brother and his now wife visited us prior to marrying (they were not specifically invited, but they were welcome) . I'm not going to tell anyone in my family they aren't welcome to visit, but they knew a visit to us meant sleeping seperately or sleeping in a motel. Sleeping together outside of marriage is against the religion that we practice in our home. I would expect any guest to be respectful of those beliefs. I never said a word to them about their choices in their own home, I expected them to respect mine as well. They slept seperately (they knew ahead that would be expected) and it wasn't an issue.
If it is an issue for you, you are right to sleep in a motel. I assume they will choose to sleep in a motel when they visit you - or they won't visit at all. You are allowed to make your own choices based on your belief system, you need to allow them to do the same.