Interesting topic, I genuinely think there may be something to this.
Keep in mind these are just my observations while in a relationship with a doggy girl.
My ex girl was zoo exclusive for about 2 years before we met, and she's a real nympho, so if anyone's body was "used to" canine semen, it was her as she's for sure had gallons of it in her over the years.
Her mood seemed like it would drop the longer it had been since she was with her boy, you could almost tell how long it had been based on how she appeared to be feeling.
There were times where she was exhausted, anxious or a bit down that were instantly improved by a quick (or sometimes not so quick) knot.
To the point I would often try and get her to take a knot before we went on a date or to some other social engagement
This didn't extend to me, her body didn't react well to birth control, and i wasn't going to force such things on her
so we primarily used condoms
the usual good sex, orgasms and lovey dovey cuddling and such as normal, but it never really brought her mood up as much or for as long as the dog did
i understand this could have just been based on her preference for dog over human, or any other number of psychological factors, rather than just her uterus slowly absorbing the dog semen during the hours afterwards.
One thing is for certain though, a good, deep 20+ minute knot, regardless of whether or not she had an orgasm, would have her beaming for days.