You are correct on this note. It doesn't. Depression is in many ways the most self consuming thing a person can face. Depending on how deep it is, the person really can't see beyond their own nose. They do good to get out of the bed in the morning, to take a shower. What's the point? If she can't even find the reason to do the things that are good for her, then why should she be concerned with those things that are for others?
The fact that she has offered you a hall pass shows that there is still some level of concern on her part to how her issues are affecting you. The hard part is that the more she sees is the more depressed she becomes. When you are severely depressed you feel like you are letting EVERYONE around you down.
Most likely she is very concerned with the lack of sex in your relationship, it's not that she doesn't care. It's that she cares so much it has immobilized her. There are so many stupid little things (I say this from experience, not because her things are stupid little things) that she feels she is failing on (things that in reality don't matter) that she can't even get to the big things. Climbing out of depression is like climbing a huge sand hill, and every time you manage to make your way a foot up the hill, you slide back down 8 inches. Eventually, you get to the point where you wonder why it's worth even bothering to try at all, so you just stop. You hide from it all, and that only makes things worse.
The best thing you can do is be there for her when she will let you. When she's available enough (mentally and physically) to spend time with you, take advantage of that time. Even it means going to the grocery store with her. When she's not available, whatever you do don't sit around sulking, it will only make her feel worse. If you have a hobby, go do it, get out of the house and let her be. There are times where she may need you to just be in the same room with her, or to just hold her, when she feels like the world is falling apart around her. She knows you care about her and love her. You are one of the few things that is holding her here and making her make the effort to find her way out of the depression.