Psychic Frank says: People have different reasons, but the better question you should be asking is why don't some men inquire about a woman's method of birth control before having relations with her? If a man suspects she is lying about being on the pill, he should use a rubber. The biological imperative to have a child transcends socioeconomic background and there are women of all stripes who get pregnant "accidentally on purpose."
I sense in you a deep regret for a missed opportunity in your youth that drives your current obsession with other women. I find that in general our society is too focused on overanalyzing past mistakes and daydreaming about the distant future, but woefully uninterested in the here and now. Too focused on what others are doing and not focused enough on their own spiritual path. For one week, I suggest a mental exercise to do nothing but focus on the present and the immediate future, and to not think or write about hypothetical other people and their choices. See how your general mood and health improve.
~ Psychic Frank