Was she always like this before marriage? If she was then you can't blame her for being asexual. You knew what you were getting yourself into.
If not, then there's probably a hidden problem. Since when did she start denying you sex? What happened around that time that might have triggered this sudden refusal to have sex?
There's several reasons. One, it could be a hormonal/biological problem and she's just not as horn y as before and isn't interested in sex, or something is stressing her out and she's too tired to have sex, or you and her got into a fight and she's still holding a grudge against you, or you did something she didn't like or that hurt her feelings while being totally unaware of what you did and whatever that is, is making her not wanna have sex with you. Gaining a lot of weight usually makes people less horn y. Are you showing her enough affection? You said you treat her right but I need to hear her side of the story.
Communication is key. Ask her why she doesn't wanna have sex, what are her reasons.
Sometimes she might feel like a sex object when you keep wanting sex. I used to have a similar mind set but my perception on sex changed. You need to change her perception of sex. Sex isn't just entertainment and pleasure, but a way to get even more closer to the one you love, to express your love and bond.
Don't ask for sex. That's a turn off. Girls love taking things slow and increasing the anticipation which makes them want it more. Just cuddle with her one night on the sofa then look deeply into her eyes and tell her you love her and some other cheesy stuff, caress her cheek and slowly lean in to kiss her, tug on her hair, move your hands over her body while making out, kiss her neck, push her onto the sofa and get on top of her, make out some more then slowly start peeling her clothes off and hopefully she won't stop you.
That should probably get you laid. But first solve the underlying issue and your current problem. Good luck.
https://www.youtube.com/watch?v=bNIKqbLuO9w