Have you thought about what would happen if china lowered the marriage age for women? 50 million+ new women in the marriage market. Problem delayed.
There's already age disparity in chinese marriages. The idea that millions of young men simply won't get married is absurd. They'll just wait longer, or expand the group of women they're willing to marry.
And I assume you're joking about chinese men marrying african women.
I don't necessarily mean marrying African women (tho' why not? historically colonists have always enthusiastically intermarried) but being out of China and working on labour-intensive projects like major infrastructure. Societies with lots of frustrated young men in them tend to go horribly wrong, Afghanistan is an example, in a society where there is a shortage of women anyway due to cultural reasons (e.g. healthcare), and successful men have 4 wives, and there is no social scene for teenagers and certainly no extramarital sex, what do the young men do? They run off into the mountains and it all goes a bit Lord of the Flies.