You're curious. That's okay and it's normal, espcially in a culture which makes it APPEAR that same-sex sex is radically different than opposite-sex sex.
(Hint: it's really not.)
(There also aren't "lines" in threesomes, where someone is the first or the third, for the record.)
But if you're grossed out by the idea of sex with her, sparing anything but kissing -- which is smething many of us so commonly do in a variety of contexts with people of all genders -- chances are good it's only curiousity, and an idea you might want to let go of, since your friend is a person, not a petrie dish.