I think Asami doesn't like other people touching what is his. I think the "what you do in the streets" comment had to do with fighting. But I think Asami will go after the person he sees as encroaching on his territory.
With the streets thing, he was actually referring Akihito's jobs honestly (which makes it weird for me when people describe in fanfics Asami bring incredibly overprotective that he doesn't allow him to pursue his career in criminal journalism. He clearly does and gives him quite a lot of freedom actually).
I think it's cheating because when Asami said he didn't care what Akihito does in the street, he means his behaviour and actions (not including his sexual relationships). Asami always thinks and says that Akihito belongs to him. He doesn't say "I love you" but he says "You're mine" and he shows Akihito his affection (and also love). I think it's tacit but it's real. And Akihito guessed that, I think. He cannot deny what he sees about Asami's feelings for him (the displays of affection).
Even if there is no word of "love", they are together so Akihito cheated on Asami. I can't wait to see Asami's reaction and anger when he discovers that, especially towards Akihito.
Surprise..surprise...
http://www.mangago.me/read-manga/you_re_my_loveprize_in_viewfinder/mh/v07/c042/9/
"I'll crush everyone who touch you" (including BJ def) xD
About the time Akihito gave that Sazaki fellow a BJ, I remember the chapter where Asami says he doesn't care what Akihito do out on the streets, and since the two of them never committed themselves to one another, their relationship, though developed, still seems somewhat casual, so it shouldn't matter if Akihito blowed Sazaki. He didn't betray or cheat on Asami in any way really