I think some actions are safe, but I also know (from a zoology perspective and a therianthropy perspective) that intercourse is painful and anxiety-inducing to a lot of animals. Even if pleasure is involved, they’ll still be expecting agony, and I know firsthand that just because something physically feels good doesn’t mean it does mentally. Besides, intercourse and merely the action of humping and/or things that mimic sex is a symbol of dominance among many mammals and that makes it emotionally uncomfortable. Plus, depending on the species, it can be dangerous for the human-bodied individual. Not just from a physical harm standpoint, but from a social and legal standpoint as well.
I’m both an (auto)zoophile and nonhuman, so this sure as hell doesn’t come from an anthropocentric standpoint. I simply think the physical and emotional risks outweigh the benefits. Plus I think you can still have a relationship with an animal without the risk of harming both of you. There are plenty of other things, like courtship, that communicate your love even better than intercourse!
i think there’s also a middle ground. intercourse can be safer and vocalizations and body language can be read to understand if your companion is uncomfortable mentally or not, even during foreplay leading up to intercourse rather than waiting until during to determine if they like it. but i do also think intercourse is straight up impossible for many couplings without causing trauma and harm, which should be avoided. outercourse and other sexual play is another story; and of course just courtship minus sex is safe.
this won’t eliminate the legal and social risks of course. but these risks are taken knowingly. for some people, the benefits of the relationship do outweigh the risk. it’s going to vary from entity to entity.