dinner party riddle, page-15

Well, I'll go out on a limb here.

2 couples were heterosexual

2 couples were lesbian

Professor shook hands with male partner of heterosexual couples i.e. 2

Then he took off to cook dinner while the rest enjoyed the evening.

After dinner, the heterosexual couples left, leaving the wife (who was unsure of her sexuality) to "entertain" four lesbians, while good hubby Professor did the dishes.

Lucky for the Professor, he got back just in time to enjoy the end of the movie, where he shook hands with himself.

Answer is 3