The comment regarding unresolved sexual issues in a marriage is right on. Left unresolved, resentment is almost a certainty, and probably on both sides as (i) the man will turn away from the woman (perhaps being passive/aggressive?), and (ii) the woman will feel as if she is only wanted for sex (because the man treats her less respectfully due to his resentment). Generally, women need to feel safe, treasured and important to be sexually motivated. The man may not make attempts to make her feel this way if he feels rejected.
It is truly a very vicious cycle - and one we are not trained to deal with. Left to our "gut feel" coping mechansisms [which may go something like "well she isn't meeting my needs, so I won't meet her needs], things usually erode. I've heard it said that a great sex life isn't an indicator of a great marriage, but a great marriage usually leads to a great sex life. To that I will add, a great marriage that suffers from a poor sex life will, over time, cease being a great marriage.
Sexual issues, like all others have to be addressed. This type of confrontation is hard, emotional and requires a man face some pretty fundamental fears. The only harder thing might be to not confront the situation; and suffer from unrealistric or unmet needs.
I'll throw this in the mix, adultry isn't an answer. It is a chicken sh** and selfish thing to do; essentially saying **** you in the strongest possible terms. Just don't do it. We need to be men. Men are responsible for thier actions. If the situation is that bad, men need to leave the relationship first - and do so with integrity.
As I sit here - totally absent from the situation the original poster is dealing with - I'll offer this parting thought: I would not expect an improvement in your sexual situation unless/until (1) the depression is effectively dealt with, and (2) your partner is motivated to change things. Only she that can change her actions; regardless of how much you may want her to - unless you are on the same page, with the same goals, it isn't likely.
I'm just some dude posting on here - with the best of intentions - and not qualified in any way to make these statements. So although "free advice" is worth every bit you paid for it - I think everyone here is very, very wll intended; otherwise we wouldn't take the time to respond.