Kairaku Onsenkyô: Nyotaiburo (1981)

An older Asian woman with short brown hair tied in a bun and wearing glasses is the standout here. She’s completely naked in a natural hot spring, sitting in the rocks with water up to her waist, getting fucked from behind by a man. The outdoor spring scene is calm but explicit, full naked bodies on display with no hiding — you see everything up close. Later, they switch to a dim bedroom where she’s on all fours, taking it hard in doggy style, her slim frame moving with each thrust. There’s also a scene with two younger Asian women, both curvy and naked, sitting at a dining table with sake bottles, laughing and half-undressed before things likely escalate. Camera stays medium close, no shaky nonsense — clear shots of faces, tits, and the actual sex.