sex, gender, nature
性 centers on the inherent qualities that define a being: its sex or gender, and its fundamental nature or disposition.
nature (of a person)
woman; female
man; male
character (of a person); personality; disposition; nature
ability; performance; efficiency
affinity; compatibility; chemistry (between people)
individuality; personality; quirk; idiosyncrasy; character; individual characteristic
sexual desire; sex drive; lust
activity
nature (of a person); disposition; temperament; character
(distinction of) sex; gender
sexual (i.e. relating to biological sex)
special characteristic; special quality; trait; idiosyncrasy; peculiarity
the opposite sex
intelligence
homosexuality; homosexual love
reason; reasoning power; (one's) sense
resistance (to heat, corrosion, etc.); tolerance (to a drug, pain, etc.)
disposition; inclination; characteristic; idiosyncrasy; propensity
high efficiency; high performance
willpower; guts; determination; grit; spirit
sensitivity; sensitiveness; sense (of ...)
same sex
sexual intercourse; sex; coitus
性 combines 忄 (heart/mind) with 生, which likely contributes both sound and the idea of life or birth, suggesting the innate qualities one is born with.
The heart radical 忄 beside life 生 points to the inner, inborn qualities of a living being: its sex and its nature.
For セイ, imagine a sage explaining the nature of all things: sage -> セイ, and the sage's wisdom reveals the true nature of life.