Eren: Would find it kind of weird but he would be happy that his sister found someone she loves. He would probably scrunch his nose and walk off but other then that he’d be fine.
Levi: doesn’t care, He would probably secretly wish he could do that with someone but in a place more private were he can relax. He would probably tell them to fix the sofa when their done.
Jean: *sad music plays in the background* He would be really depressed about it and even get a little angry. What was so good about them? What did they have that he didn’t? Eventually he would get over it and be glad Mikasa is happy.
Armin: Totally shipped them the entire time and new this was coming centuries ago. He would smile and leave them be, he wouldn’t want to disturb the lovebirds.