It is true that the prohibition of same sex marriage would be inequality, if we assume that homosexuals who say they would never be attracted to someone of the opposite sex, know themselves fully and are correct (some do experience a change whereby this can happen).
However, even if it is inequality, and I peronally think we shouldn't assume they can never love someone if the opposite sex, there are people who for legal reasons have to accept inequality of marriage. So the question is, why should we propose that homosexuals be prohibited from marrying someone of the same sex - is there good reason to deny this to them as we would do with an already married person, for example?
An already married person would be acting unjustly towards his or her spouse and children if they married another person, unless their spouse had been unfaithful.
I can only speak from personal experience. Others may have had a similar experience, and so may understand what my point is.
I once fell in love with someone who I found out was a lesbian. Basically, there was a sense of injustice that she would love a female rather than me. I felt that I had been wronged. The feeling was different from times when a girl I had been attracted to had loved another guy. In thise instances, the feeling was disappointment but not that I had been wronged.
If you asked me to articulate why I felt it was an injustice, I would have to work through the reason. But because of that experience, I believe that it is not right to allow people to marry someone of the same sex.
Maybe this is why God says that homosexual relationships should be avoided. He made marriage for opposite sex couples - I dont agree that there is any room for interpretation of the Bible otherwise