Is this what is called a Lewd Act?

I was on the late night Train from Fremantle a while back. Train has two compartments. The one I was in there was only myself and a male/female couple. 3 rows ahead, opposite aisle. Guy is sitting next to the window, female bends over into his lap and for some reason her head is bobbing up and down.🤔
Guy looks at me with a smile on his face🙂 and raises his finger to his mouth vertically asking me to keep quite.
I shrug my shoulders🤷‍♂️, and my smile is broader😁. Train is approaching a station and female raises her head and swishes her hair off her flushed face.
I have seen this happen before. About 24 years ago, same train line.