Why does the Bible call women "weaker" than men? Does this only mean physically, or is there also a spiritual or emotional inferiority?

The reference you have in mind is 1 Peter 3:7, where husbands are instructed to "be considerate as you live with your wives, and treat them with respect as the weaker partner and as heirs with you of the gracious gift of life, so that nothing will hinder your prayers." Some men have used this verse to label women inferior, and some women have felt insulted by the language.

NOT INFERIORITY

The Bible makes no mention of any "inferiority" of women to men. Inferiority denotes a lower or second-rate value, quality, or estimation. That concept of women is incompatible with biblical Christianity. "God created man in his own image, in the image of God he created him; male and female he created them" (Genesis 1:27). Adam and Eve were different and were given somewhat differing roles, but neither was superior or inferior to the other.

The shared equality of men and women in status and value did not end when mankind fell into sin. "There is no difference, for all have sinned and fall short of the glory of God, and are justified freely by his grace through the redemption that came by Christ Jesus" (Romans 3:22-24). Some have suggested that the woman sinned more than the man did. But while the Bible identifies a difference in the way Adam and Eve fell (see 1 Timothy 2:14), the guilt of Adam is highlighted in Romans 5:12-19. Since men and women also equally remain "heirs . . . of the gracious gift of life," God leaves no room for the idea that women are morally or spiritually inferior.

VULNERABILITY

Generally speaking, women are physically weaker than men. Also speaking generally, women appear specially endowed and adapted by God with an emotional capacity to express warm affection and tender care so ideal for their role as mothers and wives. Though all of us are called to be sympathetic, loving, and compassionate (see 1 Peter 3:8, the verse following the one we're looking at), women often exhibit these traits more than men do. These virtues often leave her vulnerable to abuse in this world of sinners. The abuse of women by men is a high-profile evil that permeates our fallen world.

In a marriage relationship the potentially vulnerable wife is uniquely dependent on her husband for protection and respect. Christian husbands are warned against exploiting or abusing their wives. They are instructed to treat them with respect as "weaker partners" and co-heirs of spiritual and eternal life in Christ. Domestic violence and physical, sexual, or emotional abuse have no place in a Christian marriage.