Tā xiàozhe shuō: shí sè xìng yě, hébì zhuāng qīnggāo?
He said with a laugh, 'Food and sex are human nature — why pretend to be above it all?'
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From 《孟子·告子上》 (Mencius, 'Gaozi I'), where 告子 (Gàozǐ) — not Mencius himself — argues 食色，性也 ('the appetites for food and beauty are [part of] human nature'). The 也 here is a classical sentence-final particle of assertion, equivalent to a copula.
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Classical phrasing kept intact. People drop it into speech to gently excuse a 'natural' indulgence (a hearty appetite, romantic interest), often with a knowing smile. Can sound erudite or tongue-in-cheek depending on tone.
