You have to judge each situation on it own merit. People can really justify anything they want to justify if they really put a scenario to it. There just is no clear cut correct answer. My question is the friend just a friend that happens to have a pen~s? OR is he a friend because he has one? The physchological aspect says that while platonic friendships are normal and common place, that many, if given the opportunity, will develop into a sexual situation. It's not as if the individual has premeditated openly to having sex with another person, the subconsious has decided so. Are you having troubles in your marriage? Perhaps you are seeking something your relationship is lacking. I would look past whether or not it is technically "OK" The main thing you must ask yourself is why was this person sought out as a friend to begin with? What is it in this person you find appealing. Then you have to judge.
Good Luck