I don't know what route you are playing but she is having sex with multiple people on the cuckold route, and Darrel is more central to that story cause it kicked off with him from the very first update. That is the cuckold route, not some swinger lovey-dovey route. If Ethan had a "backbone" in that route, either the route would end, or not exist at all.
And we know Ethan enjoys it, he was the one that gave the green light, even actively encouraged it and pushed Nat towards these fantasies of his. He might not be comfortable with everything Nat is doing and where it's going all the time, because Nat is starting to take her own initiatives and she is no longer purely doing this because Ethan wants too. He is no longer in complete control of what he himself set in motion. But he can't stop it now, even if he wanted too. He is a cuck with no backbone, as you put it. That is the route my friend.
Also this is a fictional porn game, stop equating Ethan and Nat's relationship to real life.
Unless you have some personal experience and would like to share with the class.