He’s in his prime
A man is only called more desirable as he ages. While a woman in her forties is considered too old to start a family, a man of that age is considered in his prime to procreate—he’s finally done playing around, and he’s established in his career.
He’s too picky
The single man doesn’t need to change—he’s just too picky.
Maybe she gets clingy
Ah, but of course. The single woman must just be clingy or crazy. She must exhibit behaviors that drive men away. Her friends must investigate that.