OR, he's a genuinely good guy that married Kaoru because he knew that she and his sister needed each other and had a special relationship and if he didn't marry her, they would eventually drift apart.
So, if at first he turned down Kaoru several times, because he didn't particularly like her, he changed his mind when he noticed how the relationship between Kaoru and his sister was evolving. It's been hinted that he actually knows something.
So, all this time, he was preparing for the future and by playing the (cheating) bad guy, he planned to fade in the background and unite Kaoru and Uta.
Who knows. I've read more complicated setups.
"I've always known my sister is in love with this girl who's obsessed with me for a long, long time."
"But first, I should marry her, fuck her countless of times, loosen her asshole and have her vagina smell like my cum for the rest of her life before I'll give her to my sister instead of being a rational human being and have them talk about their feelings."
"Now I'm gonna fuck her friend and get caught so my sister will have to heal her broken heart and they'll get closer. Which will also make both girls hate my guts! I'm so fucking kashikoi!"
Brother of the decade right there.