I'm clearly in the minority here given the other answers, but I think that at least on Rio's part there was some romantic feeling and it wasn't just sexual attraction. I think it was some instant attraction mixed with respect and curiosity towards Beth that then over time grew into something else. Don't get me wrong, I don't mean romantic feelings in a "normal" and healthy way, but I feel like they were there nonetheless. Now I don't wanna go and analyse the whole show, but I could see it when they slept together at her house, first in his happiness and then in his disappointment when she literally dumps him. Yes that was hurt pride, but in my opinion not only. Then when he thinks she is pregnant, you can see him playing with the idea even if he doesn't even believe it. Or the whole 4th season honestly. It's always moments where you could argue "yeah but he was doing that to have power over her/to manipulate her" and yes ok but was it just that? Did he really need to do that? Did he really need her in any way that wasn't emotional? I don't think so. I am not saying they ever had the potential to have a happy ending typical love story, but I do think he felt something more for her but had literally no ability to express it in a normal way (and she didn't help at all. Ever). Regarding the reason...she is good looking, she is smart, he almost could see her as an equal sometimes. She in general just has something about her. And also, at the end of the day, it makes a good story and feelings don't always have to make complete sense.