To the OP/
will: I would say there's no ethical delimna involved, as long as you are clear that you're not using sneaky sneaky coercion or psych games to get her to do something that you strongly sense she'll regret. Otherwise, she may self-identify as a lesbian, but perhaps that's not entirely accurate for her? Who can know? It's ultimately up to her...I'd sort of let tensions build up until it was obvious and ridiculous, and then have THE TALK. If it never gets there, I'd let it lie fallow.
To this other bit, I'll simply repost my thoughts from previous discussions of a similar vein:
with the addition that the very definition that sex is defined by the presence of a cock, and thus from a masculine perspective, is in my opinion incorrect. The act of (heterosexual) sex involves not only the act of penetration, but the act of being penetrated. Is there a fucker and a fuckee, or two fuckers, or two fuckees, or two people simultaneously being fuckers and fuckees. The Zen in me suggests the latter.
Regardless, carry on.