In game terms, it breaks down like this:
Before the two of you make a Fuck Test, you declare you're pullin' out right before you bust your nut and squirtin' it in a designated area of her body (we'll get to that later). Next, you make your Fuck Tests.
Regardless of whether you cum first or second you still gotta pull out before you cum. If you cum first, you pulled out BEFORE she got her "O", if you cum second, then she got hers and it's your turn.
Now to determine if you pulled your mushroom headed trout out of the pixie's sushi bar successfully you roll a number of dice equal to your Reaction. Now let's say you've got some drek hot cyberware and it gives you a Reaction of 12, well then that's 12 dice you get to roll to see if you pulled out successfully. You can also use your Seduction Skill as a Complementary Skill for more successes. To succeed you have to roll against a Target Number equal to 10 minus your Willpower, which would give a Target Number of 5. Let's roll.
You take your 12 dice (your Reaction) and roll them to get a 1, 1, 1, 2, 2, 3, 3, 4, 5, 5, 6, and a 6, 4 successes. Then you roll your Complementary Skill just to make sure and get a 2 and a 6, not enough to get an extra success, but you still made it!