As Allah created male for female and female for male.

?Holy Quran 33:35
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?Indeed, the Muslim men and Muslim women, the believing men and believing women, the obedient men and obedient women, the truthful men and truthful women, the patient men and patient women, the humble men and humble women, the charitable men and charitable women, the fasting men and fasting women, the men who guard their private parts and the women who do so, and the men who remember Allah often and the women who do so – for them Allah has prepared forgiveness and a great reward.

Holy Quran 23:5
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And they who guard their private parts.
Holy Quran 23:6
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Except from their wives or those their right hands possess, for indeed, they will not be blamed –
Holy Quran 23:7
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But whoever seeks beyond that, then those are the transgressors –
In the light of above Aayat it is Haram.According to last verse that person is transgressor therefore this act is called greater sin.