In the television series, Supernatural, S04E18, Why were Sam and Lilith going to have sex at the first place at all?
If Sam was going to have a contract with Lilith, then that would require just a kiss.
From Supernatural Wiki
As far as I remember, she explicitly tells Sam that this deal needs to be consummated instead of the usual seal-the-deal kiss. But then again, I think she was just toying with the handsome man.