Sure, the age and looks come into it, but it's not everything (as you detail well enough in your reply). More than that though, it's his actions added in to everything that make no sense for me. He's a perverted old man (not dissing that - we're probably all going to end up there, I know I am) and she's clearly disgusted by much of it and specifically telling him to stay away from her, particularly while she's using the room to express milk. Then he disregards her wishes, and continues trying to perv on her at every opportunity.
Obviously I know there's porn logic, but without that, I can't see a single thing that would make Anna look/think twice about Marvin. At the most, I suggest there might be a nice feeling at someone paying her attention, but with the stuff he does, that would quickly wear off.
It's the same the other way around as well, but you correctly note that some women are with guys who are clearly punching above their weight. There are reasons beyond looks that lead to that, but I just can't see anything about Marvin that would fit into that category. For me, I only see it working in the game if he has some sway over her (like Hank and the other old guy, the family friend), but Marvin has nothing there either, there's nothing to corrupt her with.