Hey there, and welcome to F2B
Hmm that situation is a tough one and I haven't been in a relationship at all myself so take my comments with a grain of salt here.
If it were me in this particular case, knowing full well that I wouldn't be ready to come out, I certainly wouldn't want to mislead her into a false sense of security where she thinks there's even a slight possibility of marriage or a more-than-just-friends relationship – knowing that it wouldn't lead anywhere. If it were me, I would probably tell her upfront that she's a really great friend, but that's as far as it'll go. You could give several reasons without having to lie about your sexuality like:
– I don't feel that same connection or spark that you may have for me, and with some people you immediately get that, and some people you just don't, but we can still be great friends
– I'm not looking for a relationship right now
I can't say this enough though, you should only come out when you're truly ready. For me it was make or break – ending my life or coming out, and I certainly didn't want to end my life and give up on my future, so for me I never really had that choice because it was quite literally forced upon me, but for those who are strong enough to wait it out and have that time, you'll know when to come out.
Like for me, I've come out to my friends on an online game I used to play, as well as my parents. I haven't come out to my relatives or my brother yet though until I get enough self confidence to come out to more people. But first you have to come out to yourself (where you are confident enough in who you are to the point where it doesn't matter if you lose friends or family over a small part of who you are), and sometimes staying in the closet can be a safe place until you're fully ready to come out to others. Going to a local LGBT group of some sort in your area would definitely help you feel less isolated in your circumstance.
And I don't know you personally, but from hearing countless coming out stories and being gay myself, I know that most people who are still in the closet don't want to end up marrying a person of the opposite sex only to split from them 10/20/30 years down the track. So for you, perhaps staying in the closet until you're ready to come out could be the right decision at this stage, it's up to you, and only you will have an idea as to how family, friends etc. would react to you coming out, but yeah, I think a local LGBT group would help for gaining confidence and a sense of identity, self worth and inclusion.