I’m considering cheating or leaving her.

so if you look at this, it becomes more about your feelings. if you're honest, this is what your considering leaving has more to do with. if you're not sure if you're attracted to your mate in a way that conjures up passion and romance, then its understandable why you might be questioning the relationship. But let's take it as it is, more so than how passionate she is, this is about you being unsure of how you feel anymore.

on the one hand if you care about her in a real loving way, in a way that makes you want to take care of her (love her) then the feelings are secondary. They do, with all of us, come and go somewhat, not completely dissipating but like the ebb and flow of the tides... it's not always going to be passionate and on fire. It's these times that count for the most in a relationship. when your passion is waning and you're unsure how you feel. Is she the one you would stick around for in spite of that or is that a defining moment for when it's time to leave? Your decision but just remember in every relationship this will be a factor.

true but it begs the question, is the clinical issue only on her side? In order to commit to someone that from the beginning has shown no romance kind of says something about both sides doesn't it? Why did these two commit to a gf/bf situation?

not questioning your feelings for her but I do believe, OP, that this is a question you might want to ask yourself too.