The thing you have to remember is, the manner in which women become sexually aroused is vastly different to men. Men get turned on by what they see. I.e we rely on visuals. Hence if a bloke sees a woman in a tight short skirt, or a woman in a low cut blouse, his mind goes into sexual over drive. It's also one of the reasons why we tend to watch a lot of porn, and look at porno magazines. Women are different though. They don't rely on visual stimulation as much as men do. Getting your rod out and waving it at her isn't really going to work.
Women are unique creatures. They aren't hard wired in the same way that men are. In order to get a woman really turned on you have to put in the ground work early, and you have to do it subtley. And by early I mean long before you even get her in the bedroom. Drop her little compliments. Stuff like "are those new jeans?" and she'll say "no, why?". Then you say "they suit you, you look good in them". You don't have to blow smoke up her ass, just be subtle. Just enough to make her feel desired. Women love to feel desired by their man.
Also touch. Don't be afraid to touch her. But again be subtle. Even if its just placing the palm of your hand on her lower back for a couple of seconds as you walk passed her. This will stimulate her mind. And that's the key. You have to get her mind orientated towards the thought of having sex with you. Remember, visual stimulation is not as prominent amongst women as it is in men. If I'm on the couch in the evening with my missus I'll put my arm around her, pull her in close, and ever so gently just stroke her arm with my thumb. It may appear like an innocuous show of affection, but subconsciously she will start to feel desired. I'll also smell her hair. This ground work goes in long before bed time.
The reason your missus "feels fat" is probably due to you not making her feel desired. Waving your cock at her and saying "darling I'll do the cooking and cleaning for the next month if I can ram this in you" wont make her feel desired. It will make her feel cheap. Remember, be subtle. Drop little compliments here and there. Also I hate to break this to you mate, but the primary reason women fob their men off with the excuse "not tonight, I'm not in the mood" is because the thought of having sex with you appears boring to them. Which probably stems from the fact that (and I don't mean to sound harsh here) you're just not very good at the art of love making. That is the cold harsh reality of it mate