
It curves a bit towards the woman's front. So if you are going straight in, it can actually bump against the back of the vagina before it reaches the cervix. Of course, there are a number of positions in which some portion of your penis remains outside, which is very similar to an effective reduction in length. Lifting a woman up isn't just a weird thing that people do in porn to get better film angles - it literally leaves more of the base of the penis out in the cold. That's good for filming, but if you're in trouble that too many centimeters of penis are going into the vagina, you can use that.
As for people who have given birth before: they often have looser cervices, allowing penetration into the cervix (which frees up another 20 cm or so of uterus, and I refuse to believe your penis is large enough to make that still be a problem). Of course, the act of giving birth also leads to scar tissue forming, which in some cases can make penetrative sex of any kind painful for the woman involved (there is a surgical treatment for that).
-Username17