[Butthead] Heh heh you said “penile”[BH]
Actually, despite whatever the point was of people who initially started this idea, there is something to it. Maybe I’ve got too much of my Catholic upbringing still rolling around my head, but let me see if i can present what their argument might be.
Start from this premise: throughout history, sex is PIV, solely for the purpose of forming babby. So using your hand, someone’s mouth, butthole, anything else is wrong, a sin, whatever. This goes double for animals, because they can’t consent. Bestiality in this case is just using the animal as a sperm receptacle. Might as well fuck a tree while you’re at it. There’s no “love”, no emotional connection. Just manual stimulation (and a lot of imagination).
Now, in the sci fi world things are different. Alien species are not human, but they are intelligent. Having sex with Major Kira, Dax, Deanna whatever is “close enough” that there isn’t a question. But what about Odo? Whatever his biology is, sex doesn’t involve PIV, so the non-human partner isn’t really having sex. He may not mind, but he isn’t having sex.
And what about, say, puppeteers? If you fell in love with a puppeteer, I don’t think you’re going to go put your sperm or egg inside another living creature. So sex for either of you, together, isn’t really sex. It’s going to be a compromise.
So, in a sense, alien sex is similar to bestiality, in that it’s not mutual sex. But it also isn’t bestiality, because there is no animal nature involved. Both species are able to give consent.