okay before i get yelled at i read the whole thing i dont think the solution is to dump her cause all of a sudden shes gone cold on sex she may just be pregnant and rethinking the relationship. theres a lot of reason she may be cheating on you feels guilty.
or she just like sex as much as you do.
oh i think i get this analogy but i am not sure you may need to spelll it out

Gets food>Is full>Waits>Eat more.
>Eats more anyways>Pukes.

Messin around physically>Isn't ready for further>Waits>Does More

>Does it anyways>???=realise something like pregnancy

this just happened to a friend of mine but yours might be a totally different situation.

my friend said she wasn't into the guy anymore but the sex was good so she kept going for that until now she's pregnant and don't know what to do.