I think you're very wrong on this. The reason why some feminists may be critical of a woman in a traditional heterosexual relationship is because in willingly subordinating herself to her husband, she a) puts herself in a position of dependency that might end up harming her in the future and b) reproduces those structures in the relationship which disadvantage women on a societal level, thus solidifying these structures' foothold in society. Criticism of this type of relationship isn't just about your personal preference for submissiveness, it is about you recreating society's power imbalances.
But this does not apply at all if you have a submissive relationship with a male nonhuman animal. There is no narrative in society that favours a male animal over a human woman, disadvantaging her and putting the animal in a position of power; if anything it is the reverse, and a submissive relationship to an animal is actually greatly subversive in our society's power structures.
In short, feminists wouldn't be against woman/male animal relationships since they do not reproduce a power imbalance that is disadvantaging and possibly dangerous to the woman, both individually and socially. I also doubt they would see them as degrading, because in my experience those people who are feminist will also be much more likely to question human exceptionalism and see our fellow animals as equal to us, rather than "less evolved" or inferior.
And also, for every feminist who says you should not enter a traditional hetero relationship, you'll find one who says it is completely your choice and someone who sees it ambivalently. This is by no means a decided issue, and if you think all feminists are against it, it is because you have only heard some of them speak.