NO because he might do something inappropriate
Yes it is possible
A person who is not married to a woman but has sexual relations with her is called her lover.
Sounds like your friend has a pretty bad relationship with her husband and may be staying with him for family or lifestyle reasons. Maybe she needs to know the truth to help her confront her own life and her own happiness. What about a letter, anonymous. This way she won't feel awkward with you. What makes you think she doesn't already know? If she's not having sex with him, she should suspect that he is getting it somewhere.
Yes.
Yes, it is possible to have a male best friend that is like a brother. I had one and he was my husband's best friend. There was no jealousy on my husband's part. I had met my husband's best friend years before and it was my best friend that introduced my husband and I. Try having your male friend over for dinner when your boyfriend is there and see if they hit it off. If they do then perhaps they can go out for a few beers. Don't keep them apart from each other. The more secretive you act the more suspicious and jealous your boyfriend will get. Of course, not all male/female relationships are sexual, as much as you can have a same sex friend you can have a friend of the opposite sex without any sexual feelings. In fact some people find it easier to make friends with those of the opposite sex than same sex and there is no shame in that. If someone can maintain a non-sexual opposite sex friendship it shows maturity.
It is the husband's duty to make sure his wife is pleasured sexually.
If he or she is your friend then you should treat them as such. Their sexual orientation should not enter into the matter.
CohabitationA legal description of cobabitation is: To live togeter as husband and wife. It may or may not include sexual relations. To illegally hav sexual intercourse.
If a man and wife are already married they should not go around and start two timing her/his husband/wife
That's fine and in no way makes him gay - its actually quite common when involved in sexual relations.
No. The 'Unlawful Sexual Relations' are outlined in Leviticus 18 with some listed here: Leviticus 18:13-18New International Version (NIV) 13 "'Do not have sexual relations with your mother's sister, because she is your mother's close relative. 14 "'Do not dishonor your father's brother by approaching his wife to have sexual relations; she is your aunt. 15 "'Do not have sexual relations with your daughter-in-law. She is your son's wife; do not have relations with her. 16 "'Do not have sexual relations with your brother's wife; that would dishonor your brother. 17 "'Do not have sexual relations with both a woman and her daughter. Do not have sexual relations with either her son's daughter or her daughter's daughter; they are her close relatives. That is wickedness. 18 "'Do not take your wife's sister as a rival wife and have sexual relations with her while your wife is living.
Having a sexual relationship with a friends wife no matter what race you are is unacceptable. Stay away and behave yourself. If in doubt put yourself in your friend's shoes ... in other words your friend will find out if you have sexual relations with his wife and that will be the end of the friendship which is simply not worth it.