I mean, Ino was wearing a swimsuit while they were in the bath.
If Rintarou was referring to ANYTHING he actually saw in it, it would definitely have to be the cheeks. Specially with comment on the plumpness, softness and tightness (as in, how well it keeps shape "against gravity", if I had to come up with words to express what it is meant to express).
only plumpness and tightness might be possible to apply for a butthole if you have only seen it and not touched it - reminder it was Ino washing him, then jerking him off, when they were in the bath. And they didn't have sex or anything much beyond that. And he doesn't mention touching, only seeing.