What is love? We need to define what love is. Who defines love? You? Me? Others? No. God does. God does because He is love (1 John 4:8).
The Bible talks about different types of love: agape (self-sacrificial love), phileo (brotherly love), and storge (familial love). The kind of love that most homosexuals are referring to is eros (romantic or sexual love). "Eros is the word used to express sexual love or the feelings of arousal that are shared between people who are physically attracted to one another."1 Homosexuals can rightly express agape, phileo, and storge love, however, eros, this romantic or sexual love is only blessed by God within the confines of marriage (marriage as He has defined it: one man, one woman). Eros for someone other than your spouse is sin. The Bible calls this lust (adultery of the heart). Sex outside of a heterosexual marriage is sin according to God. The Bible calls this fornication and/or adultery. The eros love that homosexuals express to one another is sin and not love (it's lust) as God has defined it.
So, why can't you marry the person that you love? Because marriage and love is only as God has defined it and "homosexual marriage" is neither marriage or love.
 "What is eros love?" (GotQuestions.org)
Also see: What Does it Mean to be In Love? (Blogos)