That varies depending on the Biblical culture you are speaking about and the time you are talking about. 'Marriage' in the Bible has far less to do with love or even sex than it did with legal and cultural traditions. At different times in the life of Israel, a man could have many wives, concubines (not legal wives, but sex partners), or at other times, the cultural tradition is to only have one wife (by the time of Jesus).
Of course, this opinion is based on a non-fundamentalist view of Scripture. This perspective sheds light on those Christian believers who understand marriage as a cultural norm and legal issue, not primarily a sexual one. In fact, Jesus (God) never even mentions anything about homosexuality, but speaks many times about heterosexual life (mostly about matters of the heart, not sexual activity).
The very few references in the Bible about homosexuality, taken only at surface level can seem to indicate that it is a huge sin. But when placed in the cultural and religious context of that time, one can see that the main point is that sexual infidelity, promiscuity and engaging in pagan rituals (which included heterosexual as well as homosexual orgies) is against God's morality for people. Combined, this brings the idea of marriage being only one man and one woman into logical question.