This is the stupidest thing I can imagine, if EA did it on purpose. I'm trying to set up a horse breeding ranch, which means having more than one mare, and one stallion to 'service' them.

And yet. The first time I have them 'try for foal'... they end up with the wedding rings and classified as 'mates'. Then my stallion won't TFF with anyone else, and neither will the mare.

WTF, seriously?

Does anybody know how to prevent this other than using the Rent-A-Stallion service at the Equine Center?

Either mine or your game is bugging because my mares and stallions can have multiple partners. The mate-wedding-ring thingy just changes to be with the latest partner (grammar?).

Do you already have the maximum number of animals on your lot (including unborn ones)?

My stallion can TFF with multiple mares, I didn't even notice there was a ring symbol to indicate they had a mate.

My stallion can also mate with multiple partners (mate status changes to new mare) BUT for some reason, my sims got the 'witnessed betrayal' moodlet, and stopped being friends with the stallion and mare #2! Funnily enough, mare #1 didn't seem that bothered...
It made me so mad, since it's just totally unrealistic. But with testing cheats, I dragged the relationships back up and got rid of those pesky (and ridiculous) moodets, so it's all fixed now

Maybe they want to remain monogamous. Not everyone has to be a whorse.