I don’t have a solid answer for you, so this is somewhat of a guess. But first, you said afterward your semen was pinkish, which of course prbly indicates some bleeding. What I want to know is, how long did that persist? Was it just the first ejaculation afterward? Or did it continue the next ejaculation or for a few days? I’d be less concerned about it if it just happened once after & did not continue.
It does sound like this activity caused some bleeding in the prostate, which produces about 40% of the total ejaculate volume, so it wdn’t be surprising to see a little blood there. However, I wdn’t think this is a common occurrence after anal sex, but maybe it is if it gets a little ‘rough’ - no experience with that myself.
Did you tell the Dr. about this sexual activity? I know it can be a little embarrassing to discuss personal sexual issues w/ a Dr., but they really need to know what type of activity you’re engaging in to give a good diagnosis. Keep in mind, they see 1,000s of patients over the years & have prbly heard just about every variation of sexual activity, so it’s not likely they’ll be embarrassed or judgmental - their job is to help you get better, but they DO need to know exactly what they’re dealing with, so be upfront about it.
Final thought - if you decide you don’t enjoy this type of sex & stop doing it & don’t see any more pinkish semen, I’d say you're prbly out of the woods & no issue with it. However, since it’s happened before, I would keep an eye open for any similar events. It’s prbly not smthg that shd keep you awake at night, just be aware to keep a lookout for it in case it happens again. If you do see continuing issues with it, please continue to see your reg. Dr or possibly a Urologist...