Myriam literally only has to imagine anything, and Zorlun already has the perfect medium to make it happen. It doesn't have to be part of the main game. In her fantasy dream realm she can take any size cock without any difficulty at all.
Hammer's cock by the way, is about twice the size ( length and girth) of any of the human cocks on offer. So they don't offer a fair comparison.
A woman's vagina is anywhere up to seven inches in length, and as a woman gets older it becomes less elastic. The anus/rectum by comparison is longer, and never really loses it's elasticity. For a woman of Myriam's age, it would be far easier to take a cock of Hammer's size in her ass. And incredibly risky for her to let him try to fuck her pussy. He would probably end up rupturing her cervix ( the narrow end of the uterus), which if it didn't kill her, would risk making her unable to conceive.