Because ultimately he’s doing what Touma wants in the hopes that he will finally listen to Kuroi and give up on Kuroi and the yakuza if Touma is appeased in this way. It makes Touma very childish in that regard because he’s likely used to Kuroi always giving in to Touma demands and expectations and this is the one situation Kuroi is resolute on not letting having Touma having his way but thought by comprising I’m sure it would sway Touma. Kuroi clearly loves Touma but it stems ultimately from a place of affection. Touma’s comes from a place of desire hence why the sex happens so quick, because Touma wants to posses Kuroi. Personally I just don’t like Touma because he’s like every other man child young seme he just has glasses and a mean glare and feels entitled to things if he demands is long enough.
*cough* not that I mind some smexy time, but didn't it feel a bit out of place? For example, I expected Kuroi to put on some resistance before it'll turn into angsty sex or something... (〜￣△￣)〜