they don't pretend to be monogamous, they Are manogamous, specifically in accordance with what Makoto mentioned in chap 4 (5?) about wanting to be with someone exclusively and finally finding that with Nekoya. And yeah, it makes no sense for a 30+ man to act like the way he does around his brother's ex, not to mention never examine the way he was groomed/violated as a child. guess that's just yaoi for you.¯_༼ ಥ ‿ ಥ ༽_/¯
the uke wasn't always like that, he started sleeping around to avoid getting cheated on by straight guys. so he wouldn't get too invested & hurt. in the chapter when he's talking to the owner of the bar it explains that. i think your confusing him getting excited by others watching with him being poly
The Uke's own brother is an open relationship with an equally kinky wife. So I don't understand why the Seme and the Uke bother to pretend to be monogamous. I thought this Yaoi would be more liberal given the story so far, why muck it up with such a tired trope like cheating? And the Uke is over 30 for fucks sake with tons of experience. Why is he acting like a maiden for his first love the fuck?