the average vaginal canal is around 5" in depth and although arousal can change that and vaginas are soft tissue, this is pretty different from giving birth.
men do typically have more rectal capacity, however, it's a pretty delicate organ overall. people who have specific interest in large insertions are "size queens" and they don't typically jump into things or do minimal prep. prep includes specially formulated highly viscous lube, butt plugs of increasing sizes over several hours, often a change in diet, and often enema rinses (so no one gets shitdick). another note is that the colon isn't straight and there's a large curve just a few inches up. this is often described as uncomfortable, especially with harder members. a bit of suppleness, which is not uncommon for massive dicks bc blood flow, definitely helps. also size queens strongly advise against numbing lubes, as this can result in missing important pain signals, leading to tearing.
additionally, there is a meme that "The average human anus can stretch up to 7 inches. The average racoon, can fit into openings, as small as 4 inches. Therefore, the average human can fit nearly 2 whole racoons up their ass."
personally, i find bottoming uncomfortable enough as it is and prefer topping by far, but especially when it comes to bigus dickus irl? terrifying

Honestly how is it even physically possible take something that huge...i ask that as a virgin . I have always been single but man the idea of having sex with a man sure scares me