I had this happen to me too and posted about it in a thread on the official forums. Someone posted they had the problem too until they figured out that proposals outside were rejected, and proposals inside a house were accepted. I don't know why that would be, but thought I'd post about it here.
Oh and I do not have Mods or CC and never have had any. So it happened for me with a completely vanilla game. My Sims were both in the green for needs, both happy/flirty (they were on vacation and had just woo hoo'ed) and the rejected proposal happened outside their tent on the campgrounds lot in Granite Falls.
Now, this all happened in a save while playing on my old toaster of a laptop that has since died, but I do still have that save file if you would like a copy? When I exited that save, the couple were back home, she sleeping in their now locked to him bedroom and he banished to a cot in the basement. He's down there crying his little Simmy eyes out. Poor little alien.
Anyway, I've had a successful proposal since getting my new desktop, (still no mods/cc) this couple were inside a house at the time though. I wasn't taking any chances that proposing outside still causes a rejection. If indeed that is what's happening for whatever reason, I have no idea. Let me know if you want that save file and how to get it to you. Thanks!
