After Uther finds out about them being into a relationship, he presses them to have sex responsibly, but they barely listen to him when he talks about this stuff, especially if it's directed to them. So Uther decides to leave leafltes of safe sex on Arthur's bed for them to find. Along with lube and condoms. Lots of them.
Here:I Give My Word to Stop at Third
Canon-era. Gwaine gets back from a long campaign.
Arthur has issues with expressing emotion and issues with admitting when he's wrong. He deals with everything with notes stuck to the bathroom mirror.
Here: Little, Yellow Post-It Notes
Merlin is a new single dad with and finds an unexpected ally in Arthur. (I'd prefer it to not be his biological child and Modern AU)
Merlin's already had one child with Arthur (who is now king) but it's a girl and Arthur needs his male heir. Arthur and Merlin have difficulty conceiving again after their daughter. Angst is great and Merlin eventually getting pregnant again is even lovelier!
The knights go all sweet and loving over their little newborn (silly faces and absurd noises). Arthur has never laughed so hard at the memory and won't let them live down with it during training.
X-Men: First Class
After Hank turns into Beast, most of his clothes don't really fit anymore so he hands them down to the other guys, although he doesn't expect tough guy Alex will want any of his "nerd" clothing. So he's surprised (and attracted) when Alex starts wearing his favorite gray cardigan 24/7.