Freebase Dali

Well over time, you're generally not as "hot" for each other as you were when you first met, given enough time. I'd say this is mostly universal, as I've never heard otherwise from anything except movies and fairytales. The timeframe most certainly varies, but it's unrealistic to think that the same feelings people have for each other, sexually, last for ever... even without the help of aging. It's just a matter of becoming accustomed to the same thing for long enough.

There are ways you can make it not as drastic, but you really have to be committed to keeping things interesting and not getting on their bad side emotionally, which is harder than you'd imagine.
In a relationship, the beginning is all glitz and glamor and you don't see the flaws (or ignore them) and you're sexually motivated by lust and "love", but it changes slowly after a while and you get more comfortable with a person and that's when the flaws (everyone has them) become apparent in both your significant other and in yourself, as reflected by your significant others' reaction to them. When the lust wears off is when you really sort of... aren't afraid... to let them know what you think, and vice versa.

I think it's just a natural thing that happens to everyone. The hard thing is trying to maintain an acceptable level of mutual attraction that's strong enough to keep you both sexually fulfilled and content. Personally I think that's a pretty important thing when you're simply a couple without kids. I wouldn't know from experience, but I'm pretty sure when kids come into the picture and you're married or whatever, they're the main focus and the attention is on keeping things together for their sake and not for the sake of your sexual urges or lack thereof.

Edit:
I also want to point out that a girl is not necessarily attracted to the way you "look" in the same way we are attracted to the way they look. Their attraction has more of a mental basis than a visual one, especially in the long term, and the way we act can have a huge effect on how attractive we are to them. Us guys might be attracted to a particular woman's body for years, but they're not so lucky if we're douchebags the whole relationship, as we'll be more of a turn-off regardless. And that directly translates to how lucky we can or cannot be, so that's an important thing to remember in a relationship, I think.