There are many beliefs about whether sex before marriage is prohibited by the Bible. I am going to avoid dealing with that issue.
I will say that if you believe that the Bible prohibits sex before marriage, that it applies to all couples whether they are same-sex or opposite-sex couples. I am a firm believer that the gender of the individuals makes no difference. All are called by God to live lives that show selfless love toward others. Forgiveness, commitment and devotion are activities not restricted to one type of relationship.
For I am convinced that neither life nor death...neither the present nor the future nor anything in all creation will be able to separate us from the love of God that is in Christ Jesus our Lord. Romans 8:38-39