Honeymoon With a 5yr Old and 3yr Old

My 5yr was very happy to have a male friend over to play the part of "husband", "prince", "king", or whatever male role she came up with that her sister can't quite fulfill (wanting to be the "princess" or "queen" or "daughter" herself). But when she announced that they were playing "honeymoon" together, I immediately wondered whether their games had gone too far. How exactly do you play such a game without it becoming inappropriate? Did I need to step in and forbid such a game? I stopped the two of them to find out what they were really playing and whether they needed to find a new activity (and whether the friend needed to never come over to our house ever again!)

Me: "What does one do on a honeymoon?"
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5yr, dressed up in skirt, high heel shoes, and a scarf (veil?) over her head: ...doesn't really know, so she avoids answering...
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3yr boy: "We're looking for a honey moon!"
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Me: "Oh, you're looking for a moon made out of honey?"
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3yr: "Yes!"
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They passed through the house several times, looking for the honey moon, searching for a ladder that would reach the moon, deciding that a search for honeybees would help them in their quest. A honey moon sounds much tastier than a cheese moon, don't you think?