Because he is having sex with "his wife" not with another married or unmarried woman. Jesus was speaking in Matthew 5 about a woman who is not his wife.
It's hard to break through the walls here... You've just given a text book example of what's known as "circular reasoning".
Again, although I think the overwhelming majority of anyone still reading this thread "gets" it... we are speaking of the
first wife... a man cannot approach another woman to have sex without violating the principle of Matthew 5 against the
first wife. She is there, perhaps in another room and he is sexually engaging another woman that is not herself... this is violating her and her ability to be made "one" with her husband.
Quite frankly, the word "authority" used here is wrong which is misleading and contrary to the "authority" of men over women mentioned in other places.
G1850 ἐξουσιάζω exousiazo - used here as authority (or power in KJV) is actually different from what you are referring here.
The only other place where G1850 occurs is:
(1Cor 6:12) All things are lawful for me, but all things are not helpful. All things are lawful for me, but I will not be brought under the power [G1850] of any.
This is not an authority. However, the best English word would be what KJV used - power. Power is not the same as authority.
1 Corinthians 7:4 is speaking about the act sex as the next verse clearly indicates not to deprive for such. As in sex, man has no power over his own body but a woman does and vice versa. This power must not be mistaken for "authority".
Here is the definition of the word in question:
1) to have power or authority, use power
a) to be master of any one, exercise authority over one
b) to be master of the body
1) to have full and entire authority over the body
2) to hold the body subject to one's will
c) to be brought under the power of anyone
Going by this definition, I stand by what I said earlier and find that the text is telling us that a man does not have the power or authority to give what is his wife's to another woman.
According to the definition of the word "exousiazo", the wife has power and may use her power over her husband's body and he has power and may use his power over her body.
The woman is the "master" of her husband's body and he is the "master" over hers. She exercises authority over his body and he exercises authority over hers.
The wife has full and entire authority over her husband's body and he has full and entire authority over hers.
His body is subject to her will and her body is subject to his.
Specifically, the power is not to be anyone, but in the context of the verse the husband exercises this power over his wife's body... and she exercises the same power over his.
Now, I agree with you that this is in the context of intimate relations between the husband and wife... but nonetheless, he doesn't have the power or the authority to give what his wife has power over to another woman.
There is just no wiggle room here. Christianity is different from Judaism. There is a new covenant and we live by it. Under the new covenant, men are not free to give their body to more than one woman.
And with this, I think I'll bow out of the discussion... it's been lively, but what more can possibly be said on this subject that hasn't been said a number of times already.