Someone who cannot say "no" - for example, because they're drunk, drugged, or knocked out - cannot say "yes"; which means that having sex with them is also rape or sexual assault. This is not just my opinion; it's written into the law in the US, the UK, and in Australia.
In Everything Changes, Owen uses an alien spray to make a woman, then a man, want to have sex with him. It's clear from the episode that neither of them wanted to have sex with him before he used the spray. We're not told how the spray works, but the Web site describes it as making the user "irresistable". If that's so, then it takes away the other person's ability to say "no" - which means that using the spray to have sex is rape. Anyone would recognise the frightening scene in Ghost Machine as date rape, but many fans don't see Owen's actions as rape or sexual assault.
One argument used in Owen's defence is that people use all sorts of artificial tricks to make themselves attractive - makeup, breast implants, flashing money around; the spray is just another trick. However, none of those tricks takes away a person's ability to say "no", which the spray apparently does.
Another argument is that the couple weren't harmed. However, we don't know whether they were harmed or not. Did all parties use protection? How were the couple psychologically affected - did they split up? If the man was straight rather than bi, is he now doubting his sexuality? Does the spray have any other side effects? We have no idea what the consequences were. It makes no difference to the legal definition of rape. (Many people believe that some kinds of rape, such as "date rape", do little harm; in fact, any kind of rape can mean serious physical and psychological damage.)
Another argument is that the scene is obviously intended to be funny - it's supposed to show Owen as naughty, not criminal. I think that's probably true; we're not supposed to hate Owen. I think it's possible the writers didn't realise the import of the scene. I think it's also possible they're developing Owen's character, first showing him using chemicals to take away someone else's ability to resist, then in the next story having him on the receiving end of that, then in the next story forcing him to empathise with a terrified rape victim - and perhaps so on. But neither explanation would alter what Owen did in Everything Changes.
How could Owen defend himself in an imaginary court case? His solicitor might argue that the spray doesn't make someone irresistable, only extremely attractive, and the couple could still have said no. Given what we see on screen, I don't think the jury would be convinced. Owen might argue that he didn't realise the spray actually made it impossible for someone to say no - he only thought it made him ultra-sexy. I think it's more likely, though, that he's just one of many people, men and women, who don't think it's rape to use chemicals to make someone have sex with you.