You're trying to predict the future, and you're assuming a lot: that long-term relationships are nothing more than a boring slog (they're not); that your friends aren't sensual outside of their partners (they might be reading erotica, too!); and that they shouldn't be satisfied with their current sex lives (they might be happy!).
As a long-term relationship evolves, your sex life might shift in ways you wind up liking - or even preferring. In many ways, I bet it will be better.
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