U -
It's not about believing you, it's hardly about trust, it's about methodology - what is true for a modded game doesn't seem approp to use as fact for the wiki. And modding one thing ~does~ change others, that's a fact as well. You've seen it, I've seen it - it cascades. The cat-tag thing is just one example.
If you say you finally got some, then... okay. How many tries? Does it seem exceptionally rare?
The whole reason I followed this up was just that - it was sitting here several days, and I was surprised no one has said "Oh, hell yes, hundreds of times!" You didn't before now, and you've been playing this game a long time.
When you buy you always get a different gender every second time. So if you buy 2 you always get one male and one female.
As i brougt 2 (m+f), the other 2 have to have been the free pair.
Your assumption proves your conclusion.
How about this for alternate theory, with assumptions that prove my theory and nothing that contradicts your experience -
1) Starting draft animals are random with regard to species.
2) If you start with 2 draft animals of the same species, you always start with 2 of the same sex.
3) If you buy more of the same species, then ~those~ are alternated with what you started with.
So, you started with two of the same sex, bought two more, and those were alternated - 2 matched pair.
Sorry, your experience isn't conclusive, but thanks anyway.