[physiqueking]

Yes; especially if it's his best friend, even more so if it's his own brother.

From: http://marriedmansexlife.com/2011/10/the...or-guests/

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The reason this happens is because before the third wheel moves in with the couple, both the third wheel and the opposite sex spouse decide whether or not they want to have sex with the other. This can happen consciously or unconsciously. Neither one of them will approve the move unless they have a positive sexual interest in the other. Then once the third wheel moves in, the sexual intentions are already essentially stated and then they just play out as expected. Two opposite sex adults sexually interested in each other, left alone in a house, with an alibi that explains why they are there together. It would be bizarre if they didn’t start fucking each other.

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The key tip off to all of this is the woman’s Isolation Anxiety. A woman will have a serious concern about ever being truly isolated with a man she does not have sexual attraction to.

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For the men in these situations, they are simply wired to take advantage of opportunistic sex when it’s presented to them on a platter. Isolation with a woman attracted to them just gets their dopamine output shooting through the roof to try and get to have sex. They might not even have a major interest in her when she moves in, but within three days they will be getting physically primed to pounce at a moments notice.