Namely, bisexual marriage. After all, who are we to say that a man who loves a man who also loves woman should not allowed to marry.
Presently there is nothing illegal about a gay man getting married if he or she can find a woman (gay or straight) who wants to marry him. There is likewise nothing illegal about a gay woman getting married to a man who wants to marry her. I'm not sure what bisexuality would add to the mix unless one allows multiple marriage. In that scenario, a man who married two bisexual women would represent a relationship that was less of an aberration than a relationship which involved two men and zero women, or two women and zero men.