Well if it were me I'd do the best friend and let the lips fall where they may. If you're considering marriage you want to know if this kind of thing is going to sway her ahead of the vows aren't you? Hell you're in a situation a lot of guys would love to be in... relax and go for it. If she develops a preferrence for female action, believe me before you go any further is when you need to know.

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As to marriage or celibacy, let a man take which course he will. He will be sure to repent - Socrates

Love is not looking in each other's eyes, but looking together in the same direction - Antoine De Saint-Exupery