As a Japanese saying puts it: Suezen kuwanu wa otoko no haji (It is shame for a man not to eat a feast placed before him).
In other words, any real Japanese man would never turn down a sexual offer from an attractive female — whether he's married or not. Adultery and infidelity are time-honored Japanese traditions — for the nation's men.
If a Japanese man wants an affair or a sexual experience outside of marriage with no strings attached, he has his choice of sexual massage parlors, "soaplands" (licensed brothels), escort services (aka "delivery health"), shady hostess clubs or numerous "meet-up" websites, which are often thinly veiled prostitution venues. If he's well-connected and wealthy, he can even register with a members-only club and be matched up with a perfect mistress.