One of the curious things about breasts is how much nipple sensitivity varies from person to person. Some get great sensual pleasure from nipple stimulation while others get nothing. And note I made no reference to gender because this can apply to either sex. The nipple nerves form during gestation and are essentially the same in both sexes. Male & female nipples are idenical until hormones kick in at puberty. Sometimes hormonal imbalance can cause male breasts to develop too- a condition call gynecomastia. (now where is Bradley??!) Male nipple sexuality is generally de-emphasized by heterosexual men because it is perceived as "gay" or "feminine" but nipples play a major part in gay sex.
Nipple rings usually enhance sensitivity by bringing stimulation closer to the nerves and most women (and men) report increased sensitivity and sensuality after piercing. But ofcourse, there are always exceptions. However as you point out- many find nip rings sexy and attractive adornments irrespective of the sensuality aspect.