First, the fact that you are concerned about their feelings speaks volumes about your approach to the LS.
Next, we always indicate that first meetings (whether from across the street, across town, or across the country) are just that--an opportunity for the two couples to get to know one another. No expectations or obligations, regardless of the distance traveled.
Finally, it is possible to be gracious and unambiguous. After a first meeting, there are only three possible outcomes:
-you never want to see a couple again;
-you would enjoy them as friends;
-you are interested in taking things further.
There are three gracious, unambiguous scripted answers:
-"Thanks for joining us for dinner, and we wish you the best on your journey."
-"Thanks for joining us for dinner. We truly enjoyed the company and conversation, and we'd like to maintain a vanilla relationship."
-"Thanks for joining us for dinner. We would like to get together with you again, eventually in a more private setting."
All three start with an appreciation for their time and effort. The first makes sure the door is closed. The second sets specific limits. The third is an invitation to the dance.
All three work.