Note: It is nice to want to keep an erection longer to satisfy your partner, however in terms of your desire to impregnate it is irrelevant whether you maintain an erection after you ejaculate (and launch those swimmers ... sperm).
SOURCES OF INFORMATION on GETTING PREGNANT
Lot of books on how to get pregnant (book stores and library). If you have difficulty with it some doctors are Fertility experts and can test a couple to be sure everything is in order for conception to occur and they too can give some suggestions.
Also, information on Internet if you do a Google search of ...
Impregnation, conception, fertility. getting pregnant