Your definition of equality is shockingly flawed. In order to try to help you understand, I will resort to the following imperfect but I hope illustrative metaphor. For your concept of marriage equality to apply, laws defining opposite sex marriage would have to stipulate that the two people involved not be in love.
The institution of marriage, until about 100 years ago, was almost exclusively concerned with arranged marriages, and in most cultures included customs regarding dowries, exchange of property, etc, etc. Only recently has marriage evolved to be between consenting adults in some places. The 'adults' part isn't required everywhere, even now.
And again, the size of a minority should not bear on providing equality for all.