I've noticed in a lot of video clips that when cocks are cumming inside a pussy, sometimes they're only doing it with just the tip in or barely the full length.
When you cum, you're supposed to shove in as deeply as you can. When you feel that change in your genitals/pelvis that tells you your orgasm is close you start shoving in and grinding. Take smaller strokes in and out of their pussy, pushing your pelvis up against theirs. Let your scrotum rest up against them and tell yourself to let go, to cum freely. Don't resist.
When you finally feel it happening and you know you're about to shoot push in as hard as you can. Don't worry about feeling any resistance. That might just be their cervix. Release right up against it and hold your body in place while your cock flexes and spurts, emptying yourself inside them.
If you feel them pushing back or squeezing your cock out that might be them cumming also! Resist and stay inside.
After you finish each throb and spurt, stay inside. Hold your tip in deeply. If you feel a secondary throb that seems alien to you that's just their cervix opening and closing, sucking up all your sperm that you've just released deeply in their pussy. You've done it exactly right.
Keep like that for awhile and don't pull out until you're starting to go soft. Keep them filled up and plugged. If you feel resistance as if there's something still pulling on your cock then it's not time to pull out yet. Until everything releases and relaxes you're still inseminating them.
Finally when things are relaxed and you're ready to come out, slide out gently and push their hips up. You don't want all that sperm you just unloaded inside them to go to waste. Don't let it drip out.
Eventually it will come out after they get up and start coming down from being filled up with your cock and cum.
If you've done everything here you've cummed in them correctly. Don't half ass it. Be committed when you ejaculate.
With any luck they'll have a positive test in a few weeks. If not, then you know what to do to try again.
