Heifer is definitely an insult, just another term for fat. And referring to your SO as "the old lady" is extremely condescending and disrespectful.
ditto!
ps: It also doesn't matter one iota how
we feel about it. bringing our feelings into it just makes it worse.
It's how your girlfriend feels about it that counts, and she's made that pretty clear.
If you go back to her and say, "Well I asked so & so, and they don't care..." you'll only dig yourself in deeper.
If you go back to her and say, "Well I asked so & so, and they thought it was an insult too, ...so I'm sorry",
...AGAIN, you're digging yourself in deeper doodoo than before. She will now be pissed off that you refused to simply accept that you offended her by referring to women as "cows", and are only willing to respect her feelings after having them validated by others. You're already in the doodoo, just simply tell her you're sorry, and that you won't do it again.