I see their relationship going to have some problems cause he was complimenting a another ...

I almost write like you did

dating couples can't feel attracted to other people? real life is going to have some problems with you. people in committed relationships still have the ability to have attractions towards people, they just won't act on it on the same principle as the vice versa. just because someone is attractive doesn't make them dating material.
i would say more that they're relationship is still unstable because his partner felt insecure from him complimenting another person

Not that you're wrong but you're completely disregarding the fact the guy only seem to date the other because of his neck. To the point it made him very insecure and had them fight sort of (something that for me, wasn't completely disproved/resolved).
Also, sure you will still feel attraction and not act on it. But usually, you should have half the brains and sensibility to not frigging comment out loud right next to the one you're dating.
Like duh.
People have feelings and it doesn't kill to be at least this mindful.

I don't disagree. Your points are the same as mine, but that wasn't the original thesis. This statement can be elaborated on much more than the post.

to be clearer, you were implying the original person who commented would have trouble in society for getting mad at this behavior. But frankly, being insensible of you're partner is just as frown upon.

your*

there's nothing wrong with being attracted to other people. But the person was obviously commenting in front of his partner. His partner who already has insecurities of being NOTHING more than a pretty nape. The fact you could do that obviously mean you care less for your partner and is only after a pretty nape.

If you care nothing more than just a person's specific feature it shows that you would go on to someone else who you see has a better feature than your current partner. So yes, their relationship is going to have problems because he complimented another girl's nape showing that he is and WILL BE easily persuaded to be withsomeone else as long as they have pretty nape. He has not shown ONCE that he prefers the uke for something other than the nape.

WHile the OP did not fully elaborate. OP does NOT NEED to because we've all read the manga, the statement is obviously meant for those who has already read the manga and should/would know what the OP means by that.