Most American doctors advise new parents to wait at least six weeks after birth before having sex. In other countries, a three-week wait is standard. In any case, your body needs some time to heal. The separation in your uterine lining caused by the expulsion of the placenta must close, and tears or episiotomy incisions have to heal and become flexible again. Six weeks might not be enough time for all of that to happen.
Numbers and norms vary among women, but research has clearly shown that breastfeeding speeds recovery from pregnancy and childbirth. A faster recovery would seem to naturally favor a more immediate return to sexual readiness.
Breastfeeding might also have an influence on how quickly women resume sex, but it's difficult to tell based on current research. One study reported that breastfeeding couples tend to wait a little longer than other couples. Another study reported that breastfeeding moms resumed sexual relations not only sooner than other moms, but enjoyed it more and experienced less pain. Yet another study agreed, suggesting that women who breastfeed return to arousability sooner than women who don't breastfeed. One thing is certain, though: Lactation is a natural part of the life cycle, and nursing helps the uterus contract, returning it to its prepregnancy size.