Be sure only the shaft, not the head of the penis comes in contact with the gal's labia.  You certainly don't want to shoot any come on there.  Let the cum go onto her stomach, or parts north of there.

Here's an even better way:  Let her grab your cock and move it to where it feels good for her, and also gives her the comfort that you won't be going anywhere verboten.  I find the touch of a gal's hand on my cock very erotic anyways.