How about one of these? They range from playful to a bit more intense:
"Going up? Because things are definitely escalating." Horny couple on stairszip
"The bedroom felt miles away, and I didn't want to waste another second." How about one of these
"Our favorite cardio happens between the first and second floor." Horny couple on stairszip
"Every step was a struggle—not because I was tired, but because I couldn't keep my hands off you."
are you going for—something more romantic or something a bit bolder?