Well, the actual problem is that Nika has HEAPS of testosterone, if we believe the removed images of his dick are still relevant, and his amazing muscle growth. It would be plausible if he had a tiny weiner and stayed a noodle - but with raging hormones, he'd need to change the sheets more than once.
Maybe the reason he jokes about dead children is he has to change his sheets after nightmares
Actually, maybe the exercise woke up the testosterone, and he'll have to change the sheets more often which will be embarrassing at Nick's place... after all the muscle mass didn't start until after college and all the hotties entered his radar. Delayed puberty?
Taking your comments more seriously, I took those kinds of statements from Nika as tongue in cheek - Nika is partially having a dig at Nami who is always looking for a sexual meaning in almost everything, she probably gets orgasms from her rolls etc. Nika not wanting to bang anyone is not ridiculous though - his problem isn't physical (as evidenced by testosterone/muscle growth) but mental - he doesn't see bodies as attractive. Consider what does get to him - scents. He notices smells and they make him feel funny, but his eyes don't care about tits or ass. His prudery with Nami in the spa is more because he isn't interested in sex and she keeps trying to push porn, talk about her fantasies, Noji's sex dungeon - he's just tired of her antics, kinda embarrassed by her. In the book club scene he makes it awkward for her, just like he does at the start when he alludes to the porn she rented. When she gave him a chub at the lake, it was physical sensation, not visual.
So yeah, he literally finds visual stimulation irrelevant. The eyes see, but the brain doesn't associate. The issue that Noji didn't find traces of ...issue... well, that's bad writing. I'm guessing Ocean meant that she didn't find tissues like Nami leaves next to her bread rolls, rather than Nika doesn't need to wash his sheets.
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