situations sounds not that fun, an affair or quick hook up can be fun but can also break what you have that is is working..
some questions to think about.
does she ever initiate? or only you?
how old is she, 40ish or much younger? can be some special cases in the 40ish age.
sex is 5 min, 10 min or 15+? does she tell to make it longer or shorter... does she starfish ever?
married double digits.. 15 years is one of the classic problem times give or take a few years.
probably your not into it, because she is not into it unless you sure you are the first one that lost interest. its not too bad yet if your still able to do and enjoy it. some get to the point that the stuff doesn't work because of lack of interest.
as you ask the question instead of just going and doing it.. might be in the category of a bit of nice guy. it is not a crime.. but many lady will be more excited by a man that is more edgy. and this might not be the case for you at at all, it is just one of the things that happens to many. the idea is that if she was really excited for you, you might also get more excited. if there is any chance of this one being part of the issue the guys hear have lots of ideas.
girl needs:
- communication, talk about stuff.
- to feel safe
lots of busy couples end up loosing the talk about stuff(man paying attention to her even if stupid topics). the lady needs to talk to a man about stuff for some time every day or every week. if not every day it needs to be alot on the days it can happen. if both are working hard and only meeting for household business and sex, the sex will get stale. eventually some other guy will talk to the girl, not right away always but eventually. Again, might not be your situation, but is one of the common things that happens to result in what you tell.
You don't think she is cheating, if she was then green light or other actions. If you want to work it out with her, try some other stuff first. I mean, would you want her to try to fix it with this idea from her side.. no judgment, this stuff is not easy.