What is it about this pic that's so hot?

This may well be cum, but the consistency of the liquid tells me otherwise.
It was tagged as ケフィア (kefir), if I recall correctly. Now I know it is often used as a cover for cum.

So, the injoke seems to stem from this CM:
http://jp.youtube.com/watch?v=AqhQlTqgicA
「それはヨーグルト...いいえ、ケフィアです。」

And applied to Pixiv:
http://www.pixiv.net/search.php?word=%E3%82%B1%E3%83%95%E3%82%A3%E3%82%A2%E3%81%A7%E3%81%99&s_mode=s_tag&x=0&y=0