Do people write thank-you notes to the parents of the groom for throwing the wedding rehearsal dinner? It seems like the right thing to do, but we don’t write thank you notes to whoever throws the wedding.
No. You do not have to write a written note to thank the groom’s parents.
A written note of congratulations and thanks (i.e. congrats on them marrying of their son and thanks for the nice dinner) is a nice gesture, but not at all expected or required.
IMHO, the only time a thank-you note is “required” is when you receive a gift.
Or when you have been a houseguest.
Or treated to a trip, or given a job lead, or suggested as a speaker… but those are all gifts, I guess.
I think the point of the OP was that being treated to a free dinner would fit in that category too.
So, while Sugaree presumably appreciates your comments, an explanation of how a dinner differs from those other gifts would also be appreciated.