Please listen to what others are saying on this thread before you do anything drastic. You think that his statement only makes sense if he had had an affair or used sex workers while you were pregnant. But that interpretation makes absolutely no sense.
Many, many people have suggested, however awkwardly he may have phrased it, that he meant he was no longer “releasing” into the baby. That’s a rather odd way of putting it, but it is (as others have said) a not uncommon feeling men have with pregnant partners. They feel like they are bumping into their own child...
His bafflement at your interpretation is consistent with this, and why on earth is someone repeatedly saying, “Where’s the evidence?” a red flag? If someone accuses you of something totally out of the blue (and frankly pretty nutty) you might very well ask where the evidence is.
I also don’t understand your statement that we are all confused because everyone has “assumptions about sex lifes etc during pregnancy”. I don’t think that is true! Some people have enhanced sex lives during pregnancy - some men find a pregnant belly and physical signs of fecundity (larger breasts, etc) very erotic. Other men become very fearful of somehow harming the baby. Some women have a strong sex drive during pregnancy. Others (especially if they are sick, or feel unattractive as their bumps enlarge) feel uninterested or unattractive.
My point is, no one is confused by the fact that sex can change during pregnancy - as it did for you as a couple.
You are obviously self aware enough to know that you may be “overthinking” and that you are “high maintenance”.
Why on earth would he point out/ blurt out / confess just when you had complimented his sexual prowess (that’s the usual meaning of “stallion” in this context) that he had spent 9 months “releasing” into someone else? Surely you can see that that makes no sense!
Why not accept what your partner says - that he was worried about hurting you and the baby before, and now that your womb is empty again he can get back to his usual desire?
I support others’ suggestion that you may not be thinking entirely clearly right now because you are still very hormonal. It might be helpful to talk this through with someone in real life, as others have suggested.
I hope things go better for you both, and your new baby, soon.