imagine whilst they live Underground, furlan and levi accidentally-on-purpose end up bathing together. and it is LITERALLY just to save water (so furlan insists, anyway) but if levi blushes throughout, then, well, that's nobody's business ;)
i imagine communal bathing is more common in the Underground with lack of plumbing and the condition of what water there is having to be filthy. imagine the middle-ages. for the water to even resemble clean, Levi’s insistent on boiling what they're going to use first, and with personal bathrooms being reserved for hostels and brothels (probably), they’d use a simple wooden basin big enough to fit them.
so maybe they use two early on. it requires extra water, but it’s worth the privacy. but THEN hell they're ALWAYS bathing together after a certain point in their friendship. clean water is a very important resource. but obviously there's no romantic/sexual tension at alllll... they basically pretend that the other isn't there. twisting around to maintain personal space.
of fucking course it all starts with a kiss and the lovebirds start fooling around in the bath. Furlan perched in Levi's lap while he suds up Levi’s hair. the goal becomes keeping the soap out of his eyes while Levi strokes them both under the water. they were already a little hard by sheer proximity, too...
Before anytime they get filthy in the sheets, it's more productive (says Levi) to make up a bath beforehand since the task is so arduous.
Cue later. Levi slotting his knees outside Furlan's hips with two of his fingers slid inside Levi, who's whispering in Furlan's ear to fuck him again. He's still open enough for it, used up from earlier. Levi, haggling in his ear to give him just one more.
And Furlan is so quick to give in; when Levi begs, it's guaranteed. Then there they are, Furlan's fingertips working into Levi's ass while he lifts himself up and down, all relaxed and tired.
but also: Levi enjoys to an adorable extent just bathing Furlan :/ It's one way he shows affection <333 cutie.