This is the first fictional heterosexual relationship that I've ever believed in.
Their love is not born out of trite longing or the common feelings of possession. They were bonded as young teens, as Beth felt responsible for the death of her mother and Rip felt responsible for the death of his family. Beth said her failing was "because I was scared", since her fear on a horseback ride caused the accident that killed her mother; Rip said his failing was "I wasn't big enough", because as a young teen, he couldn't stop his stepfather from killing his mother and siblings.
In season one, after Beth shows feelings for another cowhand, Rip tells her, "I don't care who you fuck. I just care about you." Not many men could say that. Also, when she's naked in the horse tough, he tells her to get out of it while holding his coat open and correctly says, "I'm the one thing you can't out-wrestle here." So, she gets out.
In season two, we see a lot of tender moments between them, which is all the more poignant as a contrast to the otherwise harsh nature of the show. She tells him "sorry" for having had interest in the other cowhand, and Rip sweetly says, "I'm Sorry are two words you never have to say to me."
She recognizes his feelings of love when they're sharing a wonderful evening on the roof and preempts the look in his eyes by saying, "Don't say it. It doesn't mean anything on a roof, under the stars, like a bunch of fucking hippies. Tell me when it saves me." So, the next night, when she is brutally, savagely assaulted, Rip comes to her rescue and gets her attention when she's hysterical by saying, "Hey, I love you."
It's not the kind of love that's reserved for pretty people. After the brutal assault, Beth is horribly bloodied and bruised and it shows the remainder of the season. Rip still loves her at least as much.
I've always said of my slash pairings that the sex doesn't mean anything to me unless I can believe that the characters would love each other just as much without the sex. Beth and Rip are the lone heterosexual pairing I've ever experienced that I know would love each other just as much.