Why would it be rude not to?
If Troi doesn't want to sleep with Barclay, she is in no way obligated to do so. She may have agreed to go on a friendly date with him, but there's no reason it has to go any further than that. (And Barclay, being the nice guy that he is, would never expect it to.)
However "enlightened" that 24th-century society may be, there will always be boundaries. "No" is always going to mean no.
In a lot of literary science fiction, human or alien societies are so libertine that sexual interaction is about as casual and commonplace as a handshake is to us in the early 21st century.
I think Roddenberry would have wanted Trek to lean in this direction, but he never had a chance to depict it that way.
Kor