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It sounds as if you have an uncommon skin condition called:
'Friable Urethral Meatus Opening'
This is fairly rare--and there is no 'pat' answer. But--other than the inconveniences [and sometimes discomfort], this is not a dangerous condition. It is a type of 'skin disease' that is limited to the urethral opening area. It can come on at any age [but typically between late teens to early 30s].
My suggestion is to seek out a highly-reputed Urologist or Dermatologist in your area; they will know about the condition and how to treat it.
(also-you can ask a followup Q, if you need to). "CHAT" is available too.
Best Wishes for your Health,
--Dr. Steve, M.D./FABFP
Typically--this usually happens at night [because people usually have sex at night: the resulting seminal secretions 'dry up' while one is sleeping, then attaches to the underwear like a glue; then as one moves around during sleep, it breaks the 'glue seal', causing bleed].
I suggest thorough cleansing of the penis after sex [either via shower; or 'sponge bath']; followed by some application of 'VASELINE jelly' to penis tip right before going to bed...try it: I think you'll be pleased w/ results!
"CHAT" is available too.
Should heal with proper TX; but only qualified dermatologists [usually] are adept at treating this.
It is 'safe', as long as she is completely-free of all std's.
But if the bleed is bothersome, then abstain until after you've been treated.
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