「・・・つまり、セックス！？」「うわあ、健二ったら・・・みもふたもない」

「・・・、！？」「、・・・」

'...So, you mean sex?!' 'Wow, Kenji, that's... completely tactless.'

Literal

''...In-short, sex!?'' ''Whoa, Kenji [exasperated-ったら]... without-anything-to-hide-behind''

みもふたもない ('no body, no lid') is a vivid literary idiom meaning 'blunt to the point of being tactless, brutally direct, leaving nothing to the imagination' — literally describing a container with neither body nor lid, i.e. nothing to contain the contents. The character is accused of being too direct in spelling out the euphemism. ~ったら attaches to a name as the exasperated topic particle, conveying mild frustration or fondness toward the named person. つまり ('so, in short') demands a summary.