Warn her

"I am about to cum!" I moan. Millie again whispers:

"Cum for me Matt."

And I feel my cock start to jump, and the cum shoots out of it.

Then, to my complete surprise, I feel Millie's mouth around the head of my cock, and I realise I am cuming in her mouth.

I stop jerking my cock, and feel Millie's hand instead gently stroking my shaft, milking my cum into her mouth.

I finish cuming, and I feel Millie's mouth slowly and carefully slide off the mushroom of my cock, keeping a tight seal so that none of the cum spills, then she sits up straight and makes eye contact with me.

I look at her in horny amazement as her mouth opens and I see it is full of my cum, her tongue twists around a little, playing with my cum. Millie then closes her mouth, I see her throat move as she swallows my cum, then she opens her mouth to show me she has swallowed all my cum.