You might point out that your alternative was to presume that your SIL and her fiance were sleeping together before marriage. Let us say that they aren’t…how rude would that presumption be? Even if you slept with your wife before you were married, it is more hospitable to give unmarried couples the option of sleeping in separate bedrooms without having to ask for them.
Unless you proposed to sit watch between their rooms so one couldn’t sneak over to sleep with the other–which would be insulting because even if your mistrust is well-placed, it is rude to openly imply that you do not trust them–you were not inhospitable nor rude to provide them with two places to sleep instead of one. If they knew of your beliefs and felt a need to openly tell you they wouldn’t be availing themselves of the dual accomodations, that would be rude to you. What if you knew they were already sharing a bed on a regular basis? They still might have wanted to make the pretense of sleeping in separate rooms, for the sake of the children. The possibility is not that far-fetched!
If you actually said, “I’m going to make you sleep in separate rooms”, that was rude. Again: you were implying that they would not make the moral choice without you there to force it on them. That is a rather arrogant thing to imply, to say the least, even if it is totally correct! If you made the same wrong decision that you are implying that you expect them to make, then I’d have to admit that hypocrisy is also a fair charge against you.
To use another example, let us say that you have relatives who are very messy eaters. If you invite them for dinner and inform them that you are going to make them wear bibs or even that you were going to force them to use proper table manners, that would be rude. Maybe you’d like to be able to do it, maybe their past behavior seems to warrant it, but it is out of line. Rather, as a host you are required to provide the means for proper behavior–that is, provide napkins, even if you know they’re such slobs that they never use them–and then graciously overlook their failures as much as humanly possible.
Children can be taught to understand the two principles at work here: that is, the principle of providing guests with the means to follow house rules combined with the principle that it is also hospitable to overlook the failures of guests as much as possible, too.