Yesterday I was looking after SS6 and my bio son (2 months) while DH worked.

BS was laid on his playmat on the floor, SS was playing nearby with his toys. I was in and out of the room doing general tidying up. I walked into the room they were in and SS had pulled his pj bottoms down and was squatting over my bio son with his bare bottom in my sons face. On seeing me walk in he jumped up and shot across the room while pulling his pj bottoms back up. I was on the phone to SIL at the time so told her. She found it hysterically funny whereas I found it more....disturbing.