Taking religion aside... and just focusing in on normal manners... if someone called me up and invited me out to lunch, to the movies, etc and did not invite my husband I would wonder why. ONLY a girlfriend would say "hey let's have a girls night out" ... never in a million years can I think of a single person who would ask me out and leave out my husband.
If someone asked him out and I wasn't invited, I would be insulted. There are OBVIOUS ulterior motives. I think the letter is in perfect order and if you feel you need to send it, send it off. Although I think in my house we would handle it without a paper trail, next time they called or stopped by I as the wife would say, "actually I didn't like the fact my husband wasn't invited, we are partners, a true COUPLE, and so no thank you. I only accept invitations that include my husband. Have a good day" and then hang up or close the door.
That is so rude, trying to undermine your household by sneaking in the backdoor and targeting your wife.