>>1000267an anus will be round instead of oval but have all the same problems and solutions.
Namely the problem is this, you need geometry with it's own textures and join binding and normals that look good in 2 wildly different states of deformation (2 at least). You can easily sculpt a tight butthole that looks great, or a gaping open one that also looks great. But when you deform either mesh to become the other it's very difficult. What once was big is small or vice versa. Stretching textures, now verts driven by joint A are too far away and dont look right and need to be driven by joint B.
The play is toi pick your battles. The hardest version of this problem is a fully modeled interior that must be animated and seen from very close up so it must have high res textures and impeccable normals. The easiest is like a little pocket of verts with a picture of a gaping something, and when you need to you unfold that pocket, and very quickly shove something in it, and only view it from relatively far away.
The second strategy accounts for 80% of the use cases anyways.