The 'spark' that appears to fade is not a spark at all, it is the initial getting to know you stage of every relationship when it feels incredibly exciting, it lasts about three months. Then the relationship moves into a different, better, more stable stage, when sex isn't the the top priority but family life, emotional intimacy and working for the future is. I think blaming women for this decrease of sex is unfair. the relationship is changing into a more stable and meaningful one. If your relationship is healthy, why not talk about your issues with sex with your partner, really talk, not just complain or huff about it, or worse, just avoid it altogether. ask her if you are getting her off properly and how you can make sex better for her, surpise her and remind her of the man she fell in lust with. Try other things before cheating (even if you don't consider punting cheating). Think about how devastated she would be to find out about what you do. I've spoken about this with a fair few clients in the past and one thing that came up a few times was that the partner didnt like anal, and they loved it. I'm in the same boat, my bf won't have anal sex on me, or be rough, or be in charge the way I like, he's just not that into it (at the moment anyway). so i watch porn and get off on it that way and put up with it because I love him. However because we talked about it, he now pulls my hair sometimes
things get better through communication!
Im not lecturing anyone here, I've cheated in the past myself and know how easy it is, but the long term consequences heavily outweigh the short term gains.