In Moll Flanders, sex is a kind of currency. Within marriage, and outside that institution, women like Moll trade it, whether consciously or unconsciously, for housing and food. Some women even make it their profession. Sex is definitely not always associated with love, and sometimes not even with pleasure. More often it's linked to basic survival and to power in relationships. And finally, it frequently ends in some kind of inconvenience for all those involved, especially if it's outside of marriage. For women, that's usually getting pregnant; for men, it's usually losing money.
For Moll, sex is a transaction, just like marriage.
The fact that so many of Moll's children simply disappear altogether after she gives birth to them suggests that there are no real consequences to sex in her life.