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.
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From Supernatural Wiki
Lilith is able to make deals herself and is bound by the same rules as a Crossroads Demon in making them, however, she claims a deal with her will take a sexual act instead of a kiss, but this may simply be her preference.
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.