Hi dangel! Welcome to the forum. :)
It took a lot of guts to start off with that question. I have been in the same situation, and will give you my early-morning thoughts on your question.
First, I'd like to acknowledge that you have said you are not as attracted to her -- you didn't say you have lost your attraction for her. Do you love her? I trust that you are not using this as an excuse to criticize. It doesn't sound like you are.
This happened to me about 3 yrs into my first marriage. My EX-husband had no problems telling me about his waning attraction. It really hurt my feelings, to tell you the truth. At the time, I was feeling very fat and ugly because I had gained the weight, and his comments helped me feel even worse about myself. In retrospect, I know he married me for the person he thought I was, not for the real me. I had not yet developed a strong sense of self-worth, and it cut me to the bone. He was not a pig or anything, just a young, immature man who married someone he thought was a 'catch', and very physically attactive. We all have baggage, though, and it's an amazingly freeing thing to learn to love folks for who they are, accepting the good, the bad and the ugly. I'm sure you have your not-so-attractive traits as well.
Is your wife responsible for buying groceries, cooking, cleaning, the whole 9 yards? Be honest with yourself ... Is she working outside the home and being the homemaker? This is a lot of pressure, unfair, and a huge change in her lifestyle. Many women start eating because they are literally being worked too hard, and it's the only escape they have. It's the 21st century, and we like to pretend that we are equal, yet many women have to work full time and also be the caretakers of their husbands ... maybe you should be sharing household chores in a more fair and equitable manner.
Has she given birth? Then of course she gained weight! It takes at least 1-2 years to get back into shape, if one is actively trying; don't believe what you see on TV. Some movie star who is all thin a few months after having a baby, most likely has a personal trainer, a housekeeper, and a nanny! This is the exception, rather than the rule.
Oh yes, do you spend most of your free time in front of the television or playing video games or some other hobby? Are you working 60-70 hours a week? Do you still have a full and rich lovelife (not just talking about sex here, but about the little things you did while you were dating ... phone calls in the middle of the day, bringing home a little treat, flowers, cuddling, you know what I mean). Are you still romantic? In other words, are you neglecting the 'I'm still in love' part of your relationship? This could have a lot to do with her weight gain as well. If she feels under-appreciated, she could have given up on herself, and on you.
Why don't you talk to her, and tell her you are concerned that she is gaining weight, and ask how she feels about it. I do NOT recommend telling her that you find her less sexually attractive. She may actually confide in you, if she feels secure in your love, and you may be surprised to learn that SHE is actually having a much harder time with this problem than you are. I have little doubt that she knows exactly how you feel, and it's the last thing she needs to hear from your lips! She will probably collapse inside with relief, to be able to discuss with you without feeling inadequate and stupid for gaining a few pounds.
Last, try to see her through new eyes ... as this woman you love so much. As flyin' says, think about appreciating her soft curves. Put the propaganda behind you, realize that sexual attraction begins in the mind ... and remember why you love her. Think of the feeling you get when you close your eyes and kiss her, and don't compare her to Cameron Diaz or some other impossibly thin woman who is held up in our faces every damn day of our lives, that you may find attractive.
And trust me on this one,,, it's much easier to work at your marriage than it is to start all over again. If your relationship is based on trust and respect, then it's worth working through this together. Maybe she won't ever be the slim person you married, but she can be healthy and fit, and life partnership is a wonderful thing. You are a team,,, remember that, a team.
xo
talesin