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You can (and probably should) seduce someone you've been married to for years.

Actually, the book has an entire chapter on long-term relationships and compatibility.

So how do you get into a long-term relationship if not by seducing your partner?

There are long-term relationships where sex is one of the later steps, after there's already some form of long-term commitment. There are a lot of things that can happen well before the "seduction" step -- talking, mutual interests, shared experiences, and so on.