Love and attraction is a funny thing. It does not always run in clear lines.
There used to be a straight guy on this site, a SO. His 'wife' transitioned to become a man, and they stayed together. Now he was straight. But he had a beautiful line, he said "I am not gay, but I am sure as hell glad my partner is". How lovely is that? That was love for you.
In a lot of ways the sexuality is unimportant as long as they are into you. So I would not limit it before it happens. She likes you. Keep talking. Communication is the key here. At worst, you can remain great friends.
So my advice, be happy with what it is. You like her, she likes you. As long as it is very transparant that you are a guy and you know she will always be a lesbian, and you both are into each other, and you physically hit it off, it can work. So go on, be a crazy queer couple. If it does not work out it will make for a fond memory and story. In the future you will never wonder 'what if'.
Plus think of the kudos with the guys "dude! you scored a lesbian!, your da man, seriously"

For my own part, I am a lesbian but I fell in love with a guy once. We would have been a great couple with a lot of love. There was just something between us. Who dares wins.