A sex sequence is designated by the motivation of its participants and the type(s) of sex acts that occur. See example sequence.
Each acting participant in the designated act receives a letter designating gender; then, after a space, each passive or receptive participant is designated.
Sex sequences are designated motivation, then act, then participants. Multiple acts are set apart by semicolons, with new motivations if motivations change.
In the classic feature Debbie Does Dallas, Carol is caught performing fellatio on her boyfriend in the library. The librarian, Mr. Biddle, takes her into his office; Carol believes he will tell her parents and begs him not to. Mr. Biddle proceeds to spank her.
Carol's motivations in the first sex act are mutual arousal; in the second, her motivations are to avoid an unpleasant consequence (her parents finding out about her library escapades). So we code this act like this:
(SA2 sa4 F M; SB3 sa10 M F)