It is weird indeed! If the reason is space and he needs more of it for the front seat, is ok. But to be unhappy to see a husband and wife sitting in the front seat generically is stupid. Give him a couple of chance to sit in the front alternating and make sure your husband supports you and vocalises in front of everyone asking you to sit in front. Else, just ignore and keep doing what you have to. You cant keep everyone happy all the time.
I think you should let it go. I am sure he doesn't travel with you guys on daily basis, so just ignore as I also do the same. I let my father -inlaw sit with my husband whenever we traveling together.