I think what happens is when you get married or live with the person you see each other everyday so its not as exciting anymore, your life gets very busy, plus if someone starts to put weight on you arent turned on as much anymore. I think the last one is what happens most.
Yonkers - Everyone starting out says the same things that you're saying and then 3-4 years later things change, people change. Hopefully that dosent happen to you, but from what I've seen and hear, it happens all too often.