When your own mother takes your erect penis in her hand and and guides it into her warm, wet, hairy vagina on your birthday and squeezes your balls with her hand and your penis with her vaginal muscles while you perform intercourse until you ejaculate internally and leave her a dripping wet creampie.
The predisposition to fantasize about engaging in sexual intercourse with the members of one's own immediate family group. i.e. Father, Daughter, Sister, Brother, Mother, Son. This condition is admonished by the general population but is very much alive in the minds of everyday people. At times (probably more often than not) people do enact these fantasies.
I was having sex with my gf last night and as I was cumming I said "OH MOM" and busted an incestuous load inside her!
I had an incestuous affair with my grandmother she used to give me blow jobs and let me tit fuck her!