Basically I’m wondering whether it’s totally weird or if people wouldn’t give much thought to it. I’ll use Zootopia as an example. If the main characters (a fox and a rabbit) were in a relationship, would that be really off putting?
I think people would not give much thought to it, particularly if no other characters gave much thought to it.
On the other hand, if other characters are upset about this cross-species "mixed marriage", that is going to strike close to home for people that are on both sides of the divide with regard to real-life so-called "mixed marriages" (between races, ethnicities, nationalities or religions).
If you write it so nobody even notices, you won't have a problem, it will be like a tall man marrying a short woman. Or vice versa. Or a redhead marrying a blond. Or --- like on The Big Bang Theory -- a pretty model marrying (for love) a short, insecure, glasses-wearing physicist with asthma. Unusual but unobjectionable.
The reaction will depend on your treatment.
In real life, interspecies romance is severely frowned upon and illegal in most countries. However, it is a standard trope in science fiction and fantasy (and funny animal/anthropomorphic stories, like Omaha the Cat Dancer and Albedo), and so I'd not worry too much.
If neither character is human, then I don't think any of your readers will even blink an eye (so long as both are humanoid and mammalian at least).
You could make it off-putting, as Amadeus suggests, if for example you establish foxes as predators of rabbits first and then try to introduce a relationship. That'd have some weird undercurrents. There are some extremely uncomfortable situations in Beatrix Potter stories, for example, where a pair of animals - both of whom are fully sentient and wearing clothes - are having a cup of tea together, while at the same time one of them (a predator animal) is scheming to actually eat the children of the other one (a herbivore).