Good advice. 5 years may sound like a long time, but it can take that long to heal. I know from personal experience. The first person to come along right after a serious breakup will be the next dumpee.
Never mess with a married person! he/she has already proven that he/she is untrustworthy. As was stated before, if he/she cheats on his/her spouse he/she will cheat on you.
Trust your head, your hormones will betray you.