sometimes when we have sex my boyfriends penis bleeds from just under the tip. he is not circumsized. We have let it heal then for some un known reason it will  bleed again even though we have had sex in between. It seems to happen when we do it and he is behind me
Most likely, your boyfriend has a clinical entity called frenulum breve which causes recurrent episodes of bleeding from underside of penis tip after intercourse due to a short thin anchor to the glans tip. Once in a while, the trauma of intercourse does help in that the area is replaced by a thicker fibrous scar that does not bleed, but quite often, such patients might need a simple procedure called frenuloplasty or maybe even a circumcision.