Wa `alaykum As-Salamu wa Rahmatullahi wa Barakatuh

Being naked with a women is a form of Zina by itself.

You had the ability to stop her from that act but you did not, so yes it is Zina.

And then you said, since it happened you went and did it intentionally on your own, so again it is Zina.

You need to repent with her and immediately stop such actions and never come back to it again unless in Halal through marriage.

Almighty Allah knows best.