Q:

How to make an (open) object behave like rubber when penetrated by another

So uh i've got this problem with a penetrating object and another object that serves as something like a condom if you will  
So what I want, is for the penetrating object to not deform while moving into this "rubber cylinder", while the rubber cylinder does deform and reshape itself to the outer form of the penetrating object. That is, if the pen. object is bigger than the entrance hole ofc.
Also, when the object leaves the "rubber cylinder" I want the outer edges to follow (mid object in picture) and then jump back (right object in picture)

Parenting the size and position of the "rubber cylinder"-entrance to the position of the penetrating object would work i guess, but I think it'd create problems when the pen. object enters from another angle (not perpendicular to the entrance).
I just read another post where someone mentions Vertex Weight Proximity modifier in a combination with a cast modifier. Don't know if thats gonna work yet. Couldn't find the time to test it.

A:

You could use cloth physics for the deformation part.
Pin the corners or edges of the rubber object so it keeps its shape by adding it to a Vertex Group. Let the physics do the rest.

Not sure how one could do the holes. Weight Proximity could help fake it, I don't think there is any elegant solution with Blender.
You can also assign each hole size to a shape key and manually control its diameter as necessary by keyframing the influence.

