It's a legitimate question, and I'll take only a slightly different angle on it, but I think it's an important thought to consider at this early stage in your relationship:
It's perfectly natural -- even typical -- for the endorphine-fueled, "in looove" infatuation to wane after 2-3 years. I think what you have to ask yourself is, "Is this a woman of quality and with sufficient emotional maturity that when infatuation wears off, she is capable of joining me on a marital walk where we take it to the next level (deepening emotional bond, increased shared memories and experiences, children, grandchildren, etc., etc.) and face life's challenges with me . . . or is she going to cut and run and seek the next 'high' in the rush of another affair? of drugs or alcohol? of some other short-term 'fix' of excitement?"
THAT to me is one of the underlying things about affairs you have to deal with: the whole "when the going got tough, what did she/he do?" thing.
Not easy questions to ask yourself, much less to answer I'm afraid.
