Spontaneous vs. responsive desire
The most useful framework
Sex researcher Emily Nagoski describes two fundamentally different patterns of sexual desire:
Spontaneous desire shows up without much external context. You simply feel like having sex. This tends to be more common in men, though not exclusively. People with spontaneous desire are often surprised that their partner doesn't feel the same baseline urge.
Responsive desire emerges in response to something: physical touch, an emotionally connected moment, a romantic context. People with responsive desire often don't feel like having sex until they're already engaged in intimacy. They're not broken and they're not less interested in sex; they just need a warm-up that spontaneous desire doesn't require.
Understanding which pattern applies to each partner changes almost everything about how initiation works. If the responsive-desire partner waits to feel like having sex before agreeing to it, they may rarely initiate or accept, even though they enjoy it once they're in it. The solution isn't pressure; it's a different on-ramp.