Seems like enough women have seen huge(7+ inch) dicks in porn that this is now what they expect, making it impossible for most guys to get a wife. Is this leading to the decline of white birthrates in the west?
Men are far more obsessed with dicks than women. Women mostly care about status. Pretty sure low birthrates are directly caused by the vastly increased economic status of women in the last 50 years, and it's being accelerated now by women dominating higher education and being preferentially hired and promoted in most fields.
As women's average economic status increases, the pool of higher status males is shrinking. You're not going to find some female lawyer or doctor wanting to date a welder or machinist. They'd want someone of even higher status than them. Obviously not all cases are that extreme, but if women are getting better jobs than men on average, and only want men with a job better than theirs, their potential dating pool shrinks. They keep holding out for someone better than joe average, and eventually they hit menopause and the game is up.
Even for those couples that succeed, it's extremely expensive to have a child, let alone multiple children, in countries with stagnant or declining real wages and a job market where job security is a thing of the past. How can you plan for 18+ years of expenses when you don't know if you'll still have a job in 18 months?