Try looking at her through an appreciative lens. Everyone has something about them that's hot. Did she have that sex appeal that you're looking for at one time..like, at the beginning, when you met? She had to if you added the 'benefits' part. We don't screw ones we aren't attracted to.
Anyway, what changed? Was it her or you?
It sounds like the classic 'familiarity breeds contempt' issue. I mean, if you're being tempted elsewhere already, it may not be her..maybe look at yourself?
Are you, somewhere inside, uneasy about settling down a bit..do you feel there is always something better out there? Do you feel like you deserve the BEST!?
Because listen, there is always better.
No matter whom you are with, someone else will ALWAYS be hotter, sexier, more funny, smarter, more talented, sweeter, kinder, etc..than her. And that goes for you too.
I'm sure she is exposed to plenty of men in her daily life. She easily could come across some that may impress her in some way, more than you do, (no offense!) or give her a little buzz of arousal for whatever reason.
Should that be enough for her to question YOUR sex-appeal?
At what point do you decide that what you have is pretty damn good and start feeling truly grateful? What you look for, you'll find. Look for her sex-appeal..she has it. Maybe not in the cookie-cutter way that you expect it to show up, but women have it, for sure: )
So, if you're questioning things, especially as innate as sex-appeal, you might not be the right guy for her. She can't fake that for you. You have to just see it and feel it.
Guaranteed, there are guys out there that see/know her and think "Damn, if only..."
The grass in almost never greener once you get to the other side.