I Can't Control My Desires For Him

A couple of months ago I got home early from work to find my step son fucking his girlfriend from behind on our sofa.

I must have stood there a good two to three minutes watching him thrusting into his girlfriend before they noticed me.

I made out I'd literally just entered the house, as they hurriedly got dressed. But as my step son pulled his jogging bottoms on, he made sure I saw his pussy cream covered glistening cock first.

The look we shared was pure lust both ways, and we both knew it.
When his girlfriend left, my stepson asked me how long I'd been stood there. When I lied, he said "No you weren't just there. You'd been watching us fuck for some time. I saw you in the mirror".

Before I could say anything he added as he pulled his semi hard cock out "I bet you want some of this too".
I didn't get chance to say or do anything further as his father pulled up on the driveway.
We both acted like nothing had happened and I went to have a shower.

Since then knowing in my head if my stepson, who's twenty by the way, wanted me sexually, I would have let him fuck me. And since then he's not stopped flirting with me.
Only last week he left his bedroom door open and brazenly masturbated as I walked past his room.

In eight days time his father will be going away on business for five days.
My step son has already stated he's going to fuck me when his father's not around.
I told him it's not happening. But I don't trust myself as I'm now constantly thinking about his young fit body and his gorgeous looking cock.