I adore Lois, but in most incarnations once Superman is on the scene, she doesn't notice Clark in that light for a while; she very clearly has a crush on Superman, one he reciprocates so it's easy to forgive her for overlooking the man next to her.
I can see that time being frustrating to Clark and Diana appearing as a more viable option for him to pursue in that avenue. At the end of the day, they're both great women but Lois is the one woman Clark admires, loves and respects most and will naturally end up with her. That doesn't mean the triangle can't present problems for him, especially since she's usually the first time he has to deal with balancing romance with the secret ID. With Lana, he was just himself but now he's also Superman and he's not good at that wrinkle.
The New 52 itself, however, was not a good time for our favorite reporter.
I'm not casting aspersions to any character, nor am I sticking to any one continuity. For what it's worth, I don't think it's fair to blame Lois either. Superman is an extension of Clark and she (as with the rest of the world) is caught between Clark separating those two parts of himself and compartmentalizing his life. He is the one going out of his way to pseudo test Lois, if anything it's on him. It seems born of the insecurity he sometimes has with respect to the world's perception of him as an ideal instead of a man. Blaming Lois for it is wrong, but if we're speaking from Clark's perspective, yeah. He's going to wonder if she ever loves him for himself and perhaps Diana, who wouldn't be impressed by his powers since they're normal to him, would seem like an easier path to pursue.
I'm not saying it's right, but Clark's human in all ways but literal. Young men make a lot of mistakes, especially in love. I think those wrinkles make their love story more fun to read and the end result of marriage and Jonathan much stronger because of it.
We can't pretend the classic triangle isn't problematic because it's very easy to read it as Superman gaslighting Lois until he married her, which is abusive as all hell. Much like how people read Lois having dated Luthor as confirmation that she's into power. It can go ugly quickly, but for them to really work, there does need to be conflict on the way to OTP and I think that forms very naturally in Clark struggling with the secret ID and possibly dating Wonder Woman early on.
For my money? After a short period of being starstruck, Lois realizes he's Clark and just needs to prove it. She's the world's best reporter, she can't be satisfied without sticking the story to him. The triangle becomes a game of cat and mouse between them until Clark is comfortable to finally tell her. I mapped it out in my head on a long trip once and I swear it works.