The "horses", in this case, are not really horses; more like angels in horse form, psychically bonded with good people (Heralds) to help make the kingdom safe. IIRC, there's indication that they may reincarnate, or be reincarnated Heralds, but it's never confirmed (in the books; fully read the link, and Misty has said it, and that is WOG).
So, while the Companions might accidentally share their assignations with their partners, to the partner's consternation, it's likely that the Companions know about human <ahem> relations... and might be better able to block it out if they're tired of listening to horny teenagers be horny teenagers together.