Dean and Cas are (or appear to be) cursed by heaven to kiss and fall in love and I think in one chapter Becky uses a spell to get them to make out.

I'm also 50% certain that the curse actually turns out to be that other people are manipulated by heaven to imagine Cas and Dean in intimate situations and because Cas reads their minds he gets ideas.

It was over 20k, non!au, and at one point Dean kisses Cas in a bar right after Meg and Cas kissed.

Found: Tripping by hatteress