If you and your man are two consensual adults, you are allowed to get your sex on the way you see fit. Bruising here and there may be par for your intercourse, but it can have repurcussions that go beyond your orgasms. If you roll into work with a black eye and an ace bandage fastened around your genitals people will be concerned.
Your Mom is genetically designed to care and protect you. She sees bruises on you and thinks 'fists of fury' -- not 'fists of passion'. There is only one solution: tell her the truth. If you are old enough to have sex, and rough sex at that, you are old enough to tell Mom the truth. Not because she needs to be in your sexual business, but because your sexual business found its way out of the bedroom -- on to your arm, neck, lip or front teeth.
Sure, it doesn't roll off the tongue as easy as, Mommy what's for dinner?, but it's manageable and will (*to some degree) quell Moms' fear. "It's cool, Mommy. Jeff isn't abusing me; we just get a little rowdy in bed. Also, what's for dinner?"
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Funny Guy
see... my family is just not like that lol I don't understand it because I would have no problem telling my mom what the bruises were from and then parade around with them showing, wearing them like a badge of honor, just to embarrass her... But then my mother and I are best friends and we like to embarrass, humiliate and harass one another as often as possible!
I think I have the answer for you. First tell her that your are absolutely crazy about your boyfriend and that he really loves you and cares for you. Tell her that your relationship is going very strong that that you are passionate about each other.
Then at night make audible noises when you are making love to your boyfriend so that she gets to know that your sexual relation is very intense and that your sexual needs are being taken care of.
Next day while you are sitting with your mom show your bruises to her and say, He is really passionate about me and l love it. Then ask her, "Mom, are all men as passionate?" Obviously it referes to your dad. I am sure she will shy off and understand.