There are 2 aspects to this: sexual orientation and the prospect of leaving a marriage.
In some ways the situation is no different to a heterosexual liaison- ie one person is married, meets someone else, wonders if they ought to leave marriage for the other person. But in others it's very different, because OP it's made you question your sexual identity and preferences. So you have two issues to work through, not one.
I think you ought to challenge your opinion that your marriage is 'fine'. If it were fine and dandy then you'd not be drawn so much to someone else. You need to do some really hard navel gazing over whether your marriage- with or without the appearance of this woman in your life- is all you thought it was.
Maybe it's as if you were coasting along in a Ford Mondeo, content that it 'did the job' for you, then suddenly you took a spin in a Porsche and decided 'Goodness, this is so much better!'. But whether the Porsche is a long-term good option or even available is another matter.
I've know a few women who ditched marriages for lesbian relationships in their 40s and 50s- it's not that rare.
No one sets out to have an affair- well, some might- but usually it's not premeditated. People don't set out to 'test the water' first before jumping ship. Most people meet someone by chance and , quite understandably, consider whether to leave one relationship for that person or not. It's only by meeting someone that the flaws in a relationship are sometimes put under the microscope. The hard part OP is for you to decide if this relationship has legs, or if it's a fling for the other woman, if she is as serious as you, and if you are ready to take the plunge into something new.