First up sorry if I come across as harsh it is not my attention, but I will say things as I see them.
My first observation is that you are too focused on looks.
- i'm a slim guy, wife is slim type but has been gaining fat
If she knew you put that here, on a message board, it would crush her. I hope you realize that. It doesn't matter that you are slim, and being a slim guy does not mean your wife must be also. You put it there as an excuse for your own thoughts towards her weight gain.
wife is very shy, has body image issues
Some of her current body image issues are because you are commenting on her weight gain. I don't know if you have said anything to her or not, but you don't need to. How you look at her will be enough for her to know. If you felt her weight gain was desirable and positive you would look at her in a way that communicated that and likewise if her weight gain is undesirable to you, she will know this. Having you look at her in a way that says 'you are less desirable' will each time hurt her body image and make her shyer and less likely to want intimacy. A little clue, most women are tied to their emotions. If their emotional needs are taken care of intacy will follow. If you errode her emotions she will move further and further away. So do you love her? (sorry if I missed this you have a lot of text up) If you do, try and show her this in every way possible.
- I've watched inappropriate content, I know it is sin/wrong
inappropriate content isn't just sin, it's adultery.
Matthew 5:27-28
27 “You have heard that it was said, ‘You shall not commit adultery. 28 But I tell you that anyone who looks at a woman lustfully has already committed adultery with her in his heart.
Doesn't matter that it's in a magazine or on a screen you have cheated on her with each woman you have looked at.
I got married to my wife because #1 looks was what drew me in the first place.
You should, of course, be attracted to the person you marry, but if looks were the main reason than you married for the wrong reason. You need to marry for love, for better or worse, in sickness and in health.
Ephesians 5:25
Husbands, love your wives, just as Christ loved the church and gave himself up for her
She could make the effort to work on herself; gym, diet. But it's too difficult and she would rather "enjoy" the comforts of mushrooming
The only person you can work on is yourself.
Let's reverse this for a moment to look at you, are you yourself attractive? If she came here are there other issues (besides the inappropriate content) that she would say about you? Surprising the number of men who are really not all that wanting their wives to be pretty things. For argument's sake, let's say you are, and if she isn't depressed (depressed people will also let themselves go) then my guess is she doesn't feel loved. She already knows that you don't find her desirable so why make an effort? Perhaps she also unconsciously is letting this happen so there will be less intimacy and therefore less pain.
Do you think she may have depression? I don't just mean down about her body, but actual depression that needs medical help.
Does she work? Do you verbally support her in her work? Does she feel loved, valued and supported by you? Do you take her on dates and does she make an effort to look nice on those occasions? You need to stop looking at yourself and your needs and start looking at her and her needs. Does she seem happy? keen to talk with you?
A woman who feels loved is more likely to make herself look nice, do nice things for her husband and also be more ready for intimacy. Men tend to be passion equals love while women are love equals passion.
Now about her feeling pain. This may seem a strange question, but are you circumcised? It's a little known mostly unspoken fact that some women will get pain from the circumcision scar. It kind of depends on both partners and some other factors if this happens or not, but it can. I strongly suggest you look into something called 'restoring' and 'restoring devices' and look up NORM The National Organization of Restoring Men. If you look this up you will find all kinds of information on some untold issues that circumcision can cause including painful sex and antidotal posts from men who have undergone this and the good results they have had. Not just for your wife but yourself as well.