It's not silly, it's their opinion. I bet you'd be pretty grossed out by someone making out with a cow (to continue my brown townogy from earlier) because (most likely) your morals dictate that cross-species loveual acts are gross. Some peoples' morals dictate that same-gender loveual acts are gross.
being attracted to things should always be fine, even if you can't act upon the attractions
as long as loveual actions happen between two consenting people of age it should be fine
while I personally think bestiality is gross, if there's little harm it should be fine because, you know, there's little harm
but there's probably a lot of harm considering that we vary greatly from most other animals and also we don't know how they will react